Brakes coming on uncommanded. - BMW Luxury Touring Community
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post #1 of 46 Old Aug 13th, 2019, 12:26 pm Thread Starter
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Brakes coming on uncommanded.

I was riding home musing on a thread where some were not confident of completing a journey without a breakdown. I thought to myself how I have never suffered anything more than a punctured tyre.
Stupid really as the next minute the bike started to lose power and I struggled to keep it moving. When I backed off the throttle I realised that the brakes were on even though I hadn't touched them. I came to a rapid halt and sat there wondering what had happened! I flexed the brake controls with no difference then turned off the engine and restarted it where the brakes seemed to have released.
I replaced the rear pads a couple of months ago with branded parts and have had no problems until now so don't believe that to be relevant.

Any suggestions? Caliper seems firmly attached etc.

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post #2 of 46 Old Aug 13th, 2019, 12:42 pm
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Re: Brakes coming on uncommanded.

This happened to me when a brake line burst a few years ago. As the fluid dropped to a very low level the brakes automatically came on. When I got off the bike there was no fluid left!!
At the time I didn't know the reason why but later, after installing new brake lines and while bleeding the system it happened when the fluid level went low again.
If you don't have a low brake fluid level you should check/monitor fluid level switch...........the one at the side under the seat if I remember correctly.
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post #3 of 46 Old Aug 13th, 2019, 5:19 pm
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Re: Brakes coming on uncommanded.

Quote:
Originally Posted by Scimitar View Post
I was riding home musing on a thread where some were not confident of completing a journey without a breakdown. I thought to myself how I have never suffered anything more than a punctured tyre.
Stupid really as the next minute the bike started to lose power and I struggled to keep it moving. When I backed off the throttle I realised that the brakes were on even though I hadn't touched them. I came to a rapid halt and sat there wondering what had happened! I flexed the brake controls with no difference then turned off the engine and restarted it where the brakes seemed to have released.
I replaced the rear pads a couple of months ago with branded parts and have had no problems until now so don't believe that to be relevant.

Any suggestions? Caliper seems firmly attached etc.
Wow, that is a failure mode I have not heard of before. I can’t imagine what would cause that unless there is some logic error or flaw in the ABS electronics.

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post #4 of 46 Old Aug 13th, 2019, 7:54 pm
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Re: Brakes coming on uncommanded.

That is very unusual in that the way these brakes work it cannot do it with electronics alone, there has to be a pressure input from some where. Even when the servos are running the fluid is flowing in an open circuit until a pressure input is received to push a piston forward to pinch off a ball check in the flow path. The ABS activation solenoids activate to pull that piston off of the ball check to remove the pressure preventing a wheel lock up. There must be some sort of air bubble or something that is applying pressure. See the diagrams.
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post #5 of 46 Old Aug 13th, 2019, 10:13 pm
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Re: Brakes coming on uncommanded.

Quote:
Originally Posted by jzeiler View Post
That is very unusual in that the way these brakes work it cannot do it with electronics alone, there has to be a pressure input from some where. Even when the servos are running the fluid is flowing in an open circuit until a pressure input is received to push a piston forward to pinch off a ball check in the flow path. The ABS activation solenoids activate to pull that piston off of the ball check to remove the pressure preventing a wheel lock up. There must be some sort of air bubble or something that is applying pressure. See the diagrams.
You are much more familiar with this system than me, but what would happen if the spring under the check valve failed? It seems you would still need some pressure on the control circuit transducer in order to activate the pump. It certainly isn’t obvious what combination of failures could even cause self activation.

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post #6 of 46 Old Aug 13th, 2019, 11:18 pm
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Re: Brakes coming on uncommanded.

it happened to me when the original rubber brake lines were starting to internally fail before bursting.. when applying the brake, the fluid was not able to return holding on the brakes..
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post #7 of 46 Old Aug 14th, 2019, 1:35 am
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I had that happen on my old Harley, the rear master cylinder Was gummed up. Rebuilding the MC cleared that up. I suppose that’s unlikely with a servo assist brake unit, but perhaps possible.

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post #8 of 46 Old Aug 14th, 2019, 8:18 am
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Re: Brakes coming on uncommanded.

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it happened to me when the original rubber brake lines were starting to internally fail before bursting.. when applying the brake, the fluid was not able to return holding on the brakes..
Except this isn’t what the OP described. He said he had not touched the brakes prior to them coming on. Your situation is not all that uncommon and is fairly easily explained. His situation is a real head scratcher.

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post #9 of 46 Old Aug 14th, 2019, 11:26 am
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Re: Brakes coming on uncommanded.

These are the items in the pressure chain and the movement of the chain is only 1 mm so I can understand how something could gum up the works.
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post #10 of 46 Old Aug 20th, 2019, 3:28 am Thread Starter
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Re: Brakes coming on uncommanded.

It happened again last night. I had used the brakes about a mile before the rear brake decided to deploy. Touched the rear rotor and it was hot enough to fry an egg. No amount of pumping the brake controls helped, only cycling the ignition sorted it. If I get chance, I will do a code read over the next couple of days, although I think I did it after it happened the first time, with an "all clear" result.

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post #11 of 46 Old Aug 20th, 2019, 7:02 am
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Re: Brakes coming on uncommanded.

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It happened again last night. I had used the brakes about a mile before the rear brake decided to deploy. Touched the rear rotor and it was hot enough to fry an egg. No amount of pumping the brake controls helped, only cycling the ignition sorted it. If I get chance, I will do a code read over the next couple of days, although I think I did it after it happened the first time, with an "all clear" result.
I had something similar happen to me on a vehicle I owned quite a few years back. One of the caliper pistons had stuck slightly causing the pads to contact the rotor tighter than normal which created extra heat. This apparently caused the parts to expand more causing them to become tighter with the subsequent expansion from heat. The caliper has smoking like a freight train and you could fry an egg as you said. Those brake pads were locked solid onto the rotor until it cooled down…..

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post #12 of 46 Old Aug 20th, 2019, 1:44 pm
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Re: Brakes coming on uncommanded.

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I had something similar happen to me on a vehicle I owned quite a few years back. One of the caliper pistons had stuck slightly causing the pads to contact the rotor tighter than normal which created extra heat. This apparently caused the parts to expand more causing them to become tighter with the subsequent expansion from heat. The caliper has smoking like a freight train and you could fry an egg as you said. Those brake pads were locked solid onto the rotor until it cooled down…..
Me too - on my F150 rears. Changed all the lines looking for the “collapsed” hose. Nope, it was one or both calipers that would overheat and lock the wheels. Let it sit overnight and then it would drive fine (until it overheated again).

Had a friend’s GW front overheat and lock on the interstate. We cracked the nipple to release the pressure. I thought it was from water boiling in the fluid.

Reading back through the thread, I’m not seeing anything about the last bleed or bleeding to eliminate the fluid as the cause.

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post #13 of 46 Old Aug 20th, 2019, 2:18 pm
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Re: Brakes coming on uncommanded.

The "brain" will hold all fault messages until you clear them out. Also I had an issue with "loaded" calipers for a Dodge Caravan. These were re-manufactured with pads installed. They kept dragging so I took them back to the parts house and got new calipers and new pads.

Hope you can get this sorted out.

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post #14 of 46 Old Aug 20th, 2019, 4:25 pm
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Re: Brakes coming on uncommanded.

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It happened again last night. I had used the brakes about a mile before the rear brake decided to deploy. Touched the rear rotor and it was hot enough to fry an egg. No amount of pumping the brake controls helped, only cycling the ignition sorted it. If I get chance, I will do a code read over the next couple of days, although I think I did it after it happened the first time, with an "all clear" result.
Had the same issue - riding and didn't notice that the rear brake was on (the WHINING ABS Pump and Battery light should have clued me in... ).... Anyway - got it home and it was so hot that the brake fluid was boiling/expanding out the overflow lines.

Parked (this was late last year) and now finally getting around to installing SS lines. I take it the old girl (2002) had enough of the ORIGINAL rubber lines and decided to end it all...

Yeah yeah I know - should have been replaced long ago...

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post #15 of 46 Old Aug 20th, 2019, 6:24 pm
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Re: Brakes coming on uncommanded.

Have you checked to make sure that either your front or rear brake levers aren't adjusted to the point where they are exerting a constant but slight pressure. A friend of mine put his BMW in for a service and the mechanic made some adjustments and on his ride back home his back brake locked up solid. He called his son who is also a motorcycle mechanic and by the time he got there the brakes had released. He checked the adjustment at the front brake lever and it was adjusted too close causing a constant slight pressure at the brake. After a while of course this causes heat that makes the brakes come on harder. This also probably wouldn't show up as a fault because the computer would think you were just using the brakes as normal.
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post #16 of 46 Old Aug 23rd, 2019, 5:52 am
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Re: Brakes coming on uncommanded.

I had this happen to me after performing a brake fluid change. I believe you have air inside your ABS pump. I wouldn't ride it again until you flush out the ABS controller.

My symptoms were: everything was fine until I put just enough pressure on the brake and then the ABS would run and continue to run until I turned off the bike. Then I could bring it back home with the engine friction as my main source of braking. However, if I put enough pressure on the brakes again, it would trigger the same response and recovery method. Flush the air out of your ABS pump and all should be good.

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post #17 of 46 Old Oct 2nd, 2019, 4:30 am Thread Starter
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Re: Brakes coming on uncommanded.

The 'bike went in for professional 48K service yesterday and I outlined the braking problem to them. In the afternoon I had a telephone call from the mechanic to advise me that the brake fluid was black and full of sludge. He wanted to warn me that flushing could cause issues and did I want to do it. Naturally I told him to sort it and if a can of worms develeps, then we will deal with it.

In my experience, brake fluid tends to go dark, not black and sludgy so that suggests that there are inded rubber bits breaking down.

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post #18 of 46 Old Oct 2nd, 2019, 7:14 am
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Re: Brakes coming on uncommanded.

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The 'bike went in for professional 48K service yesterday and I outlined the braking problem to them. In the afternoon I had a telephone call from the mechanic to advise me that the brake fluid was black and full of sludge. He wanted to warn me that flushing could cause issues and did I want to do it. Naturally I told him to sort it and if a can of worms develeps, then we will deal with it.

In my experience, brake fluid tends to go dark, not black and sludgy so that suggests that there are inded rubber bits breaking down.
I have had brake fluid (in a clutch, but still brake fluid) turn into almost a gelatin consistency. It was in a Jeep Comanche that was 16 years old and the fluid had never been changed. It started to get sluggish when engaging the clutch and I thought the clutch was starting to slip and going bad. I took the cover off the master cylinder and the fluid was very viscous and nearly black in color. It may well have been the hoses also deteriorating, but I flushed the stuff out and with new fluid the clutch action returned to normal and I drove the truck a couple more years before junking it.

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post #19 of 46 Old Oct 2nd, 2019, 8:20 am
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Re: Brakes coming on uncommanded.

OK...just a guess from someone with a psychology degree (my formal education), but found myself earning my living by "seat of the pants" engineering. I still consider myself relatively new to the BMW world and humbly defer to you guys with years of experience. That said, about the only explanation my tiny brain can suggest regarding brakes coming on without the driver activating the lever (creating pressure) is "THERMAL EXPANSION"(?) somewhere in the system.

I have had brake lock on trucks & cars where various conditions cause the pressure to NOT release, but never without an application (brakes applied) by the driver. Also, most brake system reservoirs are vented to atmosphere and I have seen cases where brakes can fail to operate (no brakes) if the air vent on the reservoir cap is clogged. When that happens, after warm brake fluid cools, it can cause a vacuum to develop in the reservoir and result in insufficient pressure.

So...I would be interested in the overall ambient temperature rise from the time the ride began, and the condition of the brake circuit. I could imagine that an air bubble in the system could heat up enough to expand and pressurize (activate) the brakes. I am not familiar enough with our BMW brake circuit to offer any thing more than this speculation????

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post #20 of 46 Old Oct 14th, 2019, 3:34 am Thread Starter
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Re: Brakes coming on uncommanded.

Sadly the service and brake flush achieved nothing. A couple of weeks on and the brakes applied again this morning about half a mile from where I had tested the brakes to deliberately provoke it. For some reason the dealer was reluctant to change the brake flexibles even when pressed, so I think that has to be my next operation.

My theory is not so much the one way valve, but ballooning. I wonder if the flexi is growing a bulge under hard braking, which after a short while returns to normal? The pressure can only go one way, so it goes to the caliper?

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post #21 of 46 Old Oct 14th, 2019, 9:58 am
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Re: Brakes coming on uncommanded.

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Sadly the service and brake flush achieved nothing. A couple of weeks on and the brakes applied again this morning about half a mile from where I had tested the brakes to deliberately provoke it. For some reason the dealer was reluctant to change the brake flexibles even when pressed, so I think that has to be my next operation.

My theory is not so much the one way valve, but ballooning. I wonder if the flexi is growing a bulge under hard braking, which after a short while returns to normal? The pressure can only go one way, so it goes to the caliper?
This is a perplexing one. Even though I have never heard of such a failure before, it sure sounds to me like there is some issue in the ABS that is causing the unit to think you are applying the brakes when you are not. Maybe a flakey pressure transducer or such.

Normally, when the issue is a one way restriction in a hose, the issue is that once you apply the brakes, they won’t release fully. The fact that your issue comes up when you haven’t applied the brakes suggests the problem is elsewhere and possibly thermal related with something heating up the brakes causing the pressure to build if fluid is trapped in the wheel circuits.

I don’t see any way that ballooning could cause this. You would have to have two opposing one-way valves so that pressure could be stored in the line and then later the one-way valve on the caliper side would have to leak allowing pressure to the caliper. And even in such a case, it is hard to imagine enough pressure getting built up to apply the brakes with any force.

I certainly feel your pain as the LT can be maddening at times. If you plan to keep it, then replacing the lines is worth it as even if you end up converting to manual brakes and bypassing the ABS unit, having SS lines will be a good thing.

If you check the brakes immediately after stopping when this happens, are they calipers super hot as though they have been dragging for some time, or just moderately warm such as after one normal stop?

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post #22 of 46 Old Oct 14th, 2019, 10:02 am
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Re: Brakes coming on uncommanded.

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Originally Posted by Scimitar View Post
Sadly the service and brake flush achieved nothing. A couple of weeks on and the brakes applied again this morning about half a mile from where I had tested the brakes to deliberately provoke it. For some reason the dealer was reluctant to change the brake flexibles even when pressed, so I think that has to be my next operation.

My theory is not so much the one way valve, but ballooning. I wonder if the flexi is growing a bulge under hard braking, which after a short while returns to normal? The pressure can only go one way, so it goes to the caliper?
Which ABS version do you have? I just want back through the thread and don’t see that mentioned here. a

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post #23 of 46 Old Oct 14th, 2019, 12:53 pm
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Re: Brakes coming on uncommanded.

His avatar has a face-lift model in it so it would be iABS

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post #24 of 46 Old Oct 14th, 2019, 4:08 pm
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Re: Brakes coming on uncommanded.

I know I'm just an analog guy in a digital world, but I'm still thinking that this problem is more related to the simple physics of hydraulic pressure. The trick is to discover the source of the pressure. It could be totally unrelated to the ABS system if it is some kind of debris or obstruction preventing the "release" of pressure.

The OP mentioned how hot the "rear" caliper/rotor was on one occurrence and that has me wondering if it is on the rear only, or are both calipers grabbing and not releasing? My RT is my first and only experience with a motorcycle that has ABS and I have only had a couple of times with aggressive breaking enough to give a hint of activating the anti-lock feature. So, any comment I have is purely speculative. However, my thinking is that these systems must be similar across the spectrum of bikes offering ABS and it should be of great concern to any of us using them.

In an "old-school" conventional hydraulic brake system it is a very simple rule of apply pressure to your brake lever to activate a master cylinder and it multiplies that force to the caliper cylinder that produces the braking action. The harder you squeeze...the more braking. In our ABS systems, there's the added component of "servo pumps," (looking at jzeiler's drawings, post#4). I lack knowledge and experience regarding a complete understanding of exactly how the servo's perform and control rapid adding and releasing pressure to scrub off speed without locking up the wheels.

So...I don't know if the issue is in the ABS components (servo pumps & supporting electronics) or a mechanical issue of the caliper components such as a binding piston or some piece of trash in the fluid circuit not allowing the pressure to release???

Two things I'd like to see...an immediate quick check with a no contact infrared thermometer on both wheel calipers when the brakes have bound up...and then a quick crack of the bleeders to see if there's a spurt of brake fluid & immediate release of the bound up pads?

Regardless...this is an issue we all need to follow and seek an answer to. Not to be an alarmist...but I can imagine a whole bunch of scenarios where I wouldn't want my bike to bind up & unexpectedly refuse to keep rolling.

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post #25 of 46 Old Oct 14th, 2019, 10:02 pm
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Re: Brakes coming on uncommanded.

I am of the mind that this isn't due to thermal expansion simply because there are only two ways to release that pressure. One is to allow the master cylinder to return to a point uncovering the reservoir feed hole sometimes caused by too tight an adjustment and the other would be to allow it to cool and contract. The OP said turning off the key and back on allows release so this in my mind is an electrical issue in the control systems. Key off and on would have no effect if the fluid had expanded with no return path. Going back to read again to see if any mention was made of the servo motor running before key off.

EDIT. Went back through the OP's posts on this and didn't see anywhere noticing if the servo motor was on or not when the condition was present. I might try purposefully stalling the bike to get a listen with key on, kill switch in run and issue present.
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post #26 of 46 Old Oct 14th, 2019, 11:55 pm
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Re: Brakes coming on uncommanded.

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I am of the mind that this isn't due to thermal expansion simply because there are only two ways to release that pressure. One is to allow the master cylinder to return to a point uncovering the reservoir feed hole sometimes caused by too tight an adjustment and the other would be to allow it to cool and contract. The OP said turning off the key and back on allows release so this in my mind is an electrical issue in the control systems. Key off and on would have no effect if the fluid had expanded with no return path. Going back to read again to see if any mention was made of the servo motor running before key off.

EDIT. Went back through the OP's posts on this and didn't see anywhere noticing if the servo motor was on or not when the condition was present. I might try purposefully stalling the bike to get a listen with key on, kill switch in run and issue present.
You have challenged me to do more digging. If this is a malfunction of the ABS, then (after a bit of online study of ABS tech), I'm now thinking it must be somewhere in the area of the speed sensor and how it initiates the servo to activate. (?) As I understand it, the servo's are mainly activated when braking pressure and speed detect a potential wheel lockup and therefore begins the rapid sequence of pulsing the brakes. In this case, if the brakes begin to pressurize without the rider operating the lever...then some sensor is kicking in the servo and it is pressurizing but not pulsating???

So, whatever signal that is supposed to operate the "release" portion of the pulsing action is not happening. Of course, if there's no real reason for "anti-lock" action going on in the first place...then something haywire (hillbilly technical term) is going on electronically.

It would be great if someone who really has the equipment and knowledge to give the system a "static" test that would put it through it's paces without risking life & limb could diagnose and explain exactly where the fault lies. I'm already extended beyond my capabilities...so I'm gonna slink back into lurking mode and await someone to enlighten us.

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post #27 of 46 Old Oct 15th, 2019, 6:55 am Thread Starter
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Re: Brakes coming on uncommanded.

The fault seems to affect only the rear brake. The first symptom is that the power seems to drain which suggests that it is being progressively applied. Often, the brake heat can be smelled, let alone felt. I have not listened for servo pump as I have a helment on with "sounds". Good call that though.

It is a 2004 onwards facelift model.

GS-911 does not show any fault so in my mind, the ABS is happy. That points to something purely mechanical.

The problem only shows after I have used the brakes to stop fairly quickly. The problem then develops after half a mile's travel or so. There is no apparent drag until it decides to apply and then it is coming to a stop, like it or not.

Yesterday I cycled the ignition quickly, which achieved nothing. I had to count to ten before restarting and then it was back to normal.

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post #28 of 46 Old Oct 15th, 2019, 9:06 am
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Re: Brakes coming on uncommanded.

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The fault seems to affect only the rear brake. The first symptom is that the power seems to drain which suggests that it is being progressively applied. Often, the brake heat can be smelled, let alone felt. I have not listened for servo pump as I have a helment on with "sounds". Good call that though.

It is a 2004 onwards facelift model.

GS-911 does not show any fault so in my mind, the ABS is happy. That points to something purely mechanical.

The problem only shows after I have used the brakes to stop fairly quickly. The problem then develops after half a mile's travel or so. There is no apparent drag until it decides to apply and then it is coming to a stop, like it or not.

Yesterday I cycled the ignition quickly, which achieved nothing. I had to count to ten before restarting and then it was back to normal.
OK, thanks for the clarification. This is a totally different situation than what your initial post had me thinking. So, really...the braking action is not some mysterious un-initiated happenstance. It is your bike and your money but...(If you think they have been damaged too much by being overheated.) my response would be to completely replace the rear caliper and pads. At least, a complete rebuild (seals, etc.) flushed and new pads, and a thorough bleeding.

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post #29 of 46 Old Oct 15th, 2019, 9:29 am
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Re: Brakes coming on uncommanded.

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I am of the mind that this isn't due to thermal expansion simply because there are only two ways to release that pressure. One is to allow the master cylinder to return to a point uncovering the reservoir feed hole sometimes caused by too tight an adjustment and the other would be to allow it to cool and contract. The OP said turning off the key and back on allows release so this in my mind is an electrical issue in the control systems. Key off and on would have no effect if the fluid had expanded with no return path. Going back to read again to see if any mention was made of the servo motor running before key off.

EDIT. Went back through the OP's posts on this and didn't see anywhere noticing if the servo motor was on or not when the condition was present. I might try purposefully stalling the bike to get a listen with key on, kill switch in run and issue present.
That was my initial thought also, but after John posted the diagrams, this seems unlikely as the electronics can control the servo to pump fluid, but, if I understand the diagrams correctly, this is just circulated through the wheel circuit reservoir as the ball valve is open. And it seems to me that the electromagnet can only open the ball valve and not close it. Is that correct, John? It seems that only pressure in the control circuit can close the ball valve and thus allow the servo pump to build pressure in the wheel circuit. So you would need a failure in the control circuit that allows the ball valve to be pushed closed without any activation of the control level. That seems pretty unlikely.

Although I am not sure yet that I fully understand the diagrams as something looks odd to me with integral circuit. The diagrams appear to show that the integral piston from one wheel is using fluid into the control circuit of the other wheel. However, if that level is not activated, then the master cylinder port should be uncovered and the pressure would simply push fluid into the master cylinder reservoir. So, it seems there must be at least one more valve in the system that is between the master cylinder and where the fluid from the integral valve is injected. And it seems this would have to be a controlled valve as if it was a simple check valve then the pressure from normal lever activation would also be trapped in the control circuit and the brakes could not release. So, it seems to me that the diagrams are still missing some detail. Is that detail important to this particular failure mode? I have no idea.

And having just read the OPs last post about the problem arising shortly after a hard stop and key activation not immediately correcting the problem and having to wait a short time, that leads me back to it being potentially a thermal problem with fluid trapped in the wheel circuit. If it was my bike, I would change all of the brake lines and thoroughly bleed the system myself to ensure that the complete 1-2-3-1 procedure was followed to the letter.

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post #30 of 46 Old Oct 15th, 2019, 9:43 am
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Re: Brakes coming on uncommanded.

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Originally Posted by Voyager View Post

And having just read the OPs last post about the problem arising shortly after a hard stop and key activation not immediately correcting the problem and having to wait a short time, that leads me back to it being potentially a thermal problem with fluid trapped in the wheel circuit. If it was my bike, I would change all of the brake lines and thoroughly bleed the system myself to ensure that the complete 1-2-3-1 procedure was followed to the letter.
If simply sitting for 10 seconds does not releave the brake pressure and it requires de-energising the electronics to correct, there has to be some electronic component involved this issue even if it is a mechanical component which is why I asked if the servo motor is running or not. If it isn't running, then I am not sure why turning the key off would have any effect on the issue.
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post #31 of 46 Old Oct 15th, 2019, 11:06 am
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Re: Brakes coming on uncommanded.

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If simply sitting for 10 seconds does not releave the brake pressure and it requires de-energising the electronics to correct, there has to be some electronic component involved this issue even if it is a mechanical component which is why I asked if the servo motor is running or not. If it isn't running, then I am not sure why turning the key off would have any effect on the issue.
Except, if I read his last post correctly, he immediately cycled the power and it did nothing. If it was purely electronic, that should have resolved it. It seems something else is at play here and it isn’t purely an electronic issue as it appears time is involved in the resolution also.

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post #32 of 46 Old Oct 15th, 2019, 2:07 pm
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Re: Brakes coming on uncommanded.

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Except, if I read his last post correctly, he immediately cycled the power and it did nothing. If it was purely electronic, that should have resolved it. It seems something else is at play here and it isn’t purely an electronic issue as it appears time is involved in the resolution also.
It could require capacitive discharge time

Not saying it isn't mechanical in nature but it seems to have an electrical component or the key would have no effect. Probably the rear control circuit

I think we need to know that missing bit of data, is the servo motor running? At least that would point us in some direction.

The steps I would take to isolate it further when in the failure mode are,

1. When it happens, stall the bike and listen for servo noise.
2. Raise the bike on the center stand if capable and see if it is centrist to front or rear ( yes, they are integrated so is it one or both wheels. )
3. Verify that at key off, the wheel(s) do release and not before.
4. Replace all lines and flush thoroughly if still OEM
5. Check the rear pedal adjustment to make sure it has proper free play and is not closing off the replenishment orifice.
6. Check rear brake switch adjustment and function.

John Zeiler, is the brake switch what energizes the servo motor or is it a pressure transducer or both.

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post #33 of 46 Old Oct 15th, 2019, 5:07 pm
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Re: Brakes coming on uncommanded.

It is both switch and/or pressure that triggers the servos. The anti lock solenoid only removes the check ball if there is pressure pushing it in, it cannot push the ball closed.

I would be interested to know if the rotor is stock as well as the rear pads and any other history of maintenance there (cow bell repair?).

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post #34 of 46 Old Oct 16th, 2019, 3:38 am Thread Starter
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Re: Brakes coming on uncommanded.

The problem didn't show until I replaced the rear brake pads with branded parts from a BMW specialist. Nothing else had been done to it in my ownership. The pads were almost down to the backing.
I had to remove the caliper to do the job, hence my ideas about the rear flext breaking up, but all that was double checked during the professional service in the last few weeks. The brake system was full of black and sludgy fluid but they didn't spot anything else untoward.

I tried to provoke it last night so I could check the servo status, but it didn't lock up this time.

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post #35 of 46 Old Oct 16th, 2019, 4:39 pm
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Re: Brakes coming on uncommanded.

OK well I hope you sprayed cleaner on the pistons before you pushed them back in or you may have scored the bores or gotten dirt jammed in there. This could cause the pistons to stick a bit and then cause the pads to heat up and then cause the fluid to expand and apply more pressure. Just a guess though.

Also the bolts each have a unique place to go they are not interchangeable, If you get it wrong one will rub the rotor.
The rounded one goes on the rear most hole with the support for the ABS sensor
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post #36 of 46 Old Oct 16th, 2019, 8:49 pm
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Re: Brakes coming on uncommanded.

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Wow, that is a failure mode I have not heard of before. I can’t imagine what would cause that unless there is some logic error or flaw in the ABS electronics.
Same here, never hear of it before..

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post #37 of 46 Old Oct 17th, 2019, 4:14 am Thread Starter
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Re: Brakes coming on uncommanded.

I tried again this morning. Pulled up fairly hard and waited for the problem to appear. It didn't.

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post #38 of 46 Old Oct 17th, 2019, 8:35 am
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Re: Brakes coming on uncommanded.

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I tried again this morning. Pulled up fairly hard and waited for the problem to appear. It didn't.
Have you replaced those brake lines yet? If you are running the originals, 15 years is a long time for the OEM hoses.

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post #39 of 46 Old Oct 17th, 2019, 12:24 pm Thread Starter
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Re: Brakes coming on uncommanded.

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Have you replaced those brake lines yet? If you are running the originals, 15 years is a long time for the OEM hoses.
Not as yet. My searches have not turned up anything compatible in the UK. Goodrich is the name for SS brake lines here, but they don't list a late LT. Speigler is an option if the import cost wasn't stratospheric!

I will replace them as soon as I can, they have been under suspicion from the start.

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post #40 of 46 Old Oct 17th, 2019, 12:35 pm
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Re: Brakes coming on uncommanded.

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Not as yet. My searches have not turned up anything compatible in the UK. Goodrich is the name for SS brake lines here, but they don't list a late LT. Speigler is an option if the import cost wasn't stratospheric!

I will replace them as soon as I can, they have been under suspicion from the start.
Based on all you have posted thus far, I think you really want to consider replacing the lines before doing a lot more troubleshooting. I really doubt the issue is electronic. I suspect it is either a blockage in the line or a sticking piston in the caliper as John mentioned. Going from complete worn our pads to brand new requires a lot of travel in the piston as if they aren’t cleaned well before being pushed back into the bore, bad things can happen. I am not a big fan of using brake cleaner as it can be very rough on rubber and polymer seals. My preference is to use Dawn dish soap and a toothbrush. That will remove most dirt and other abrasive materials and I have yet to have a piston stick or score a bore using this method. It is messier, especially if you need to work inside, but it also allows you to thoroughly clean the caliper also which is nice.

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post #41 of 46 Old Oct 17th, 2019, 5:09 pm
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Re: Brakes coming on uncommanded.

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Based on all you have posted thus far, I think you really want to consider replacing the lines before doing a lot more troubleshooting. I really doubt the issue is electronic. I suspect it is either a blockage in the line or a sticking piston in the caliper as John mentioned. Going from complete worn our pads to brand new requires a lot of travel in the piston as if they aren’t cleaned well before being pushed back into the bore, bad things can happen. I am not a big fan of using brake cleaner as it can be very rough on rubber and polymer seals. My preference is to use Dawn dish soap and a toothbrush. That will remove most dirt and other abrasive materials and I have yet to have a piston stick or score a bore using this method. It is messier, especially if you need to work inside, but it also allows you to thoroughly clean the caliper also which is nice.
I am still curious why a key off even if you have to count to 10 has any effect with a possibly blocked line. Normally it would have to cool and contract to get relief otherwise simply sitting still for 10 seconds should produce the same result. Maybe we will get lucky and it will fail again and we can get a read on the servo motor.

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post #42 of 46 Old Oct 17th, 2019, 7:04 pm
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Re: Brakes coming on uncommanded.

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I am still curious why a key off even if you have to count to 10 has any effect with a possibly blocked line. Normally it would have to cool and contract to get relief otherwise simply sitting still for 10 seconds should produce the same result. Maybe we will get lucky and it will fail again and we can get a read on the servo motor.
Well... You are focused on the "Electronic" possibilities, and others of us leaning more toward the mechanical Issues...when I first read the initial post, I was very alarmed at even the hint of a possibility of the brakes applying themselves without any action by the driver!

However, as this conversation has evolved, the story has changed and I am no longer as alarmed but my curiosity remains. I've been thinking about it and (for me) between my hand lever and foot lever, braking is almost as "autonomic" as taking a breath or blinking the eyes. So, to make an absolute claim my brakes came on and pressurized with no input from me I would have to be operating on an interstate slab, relaxed, with cruise control engaged and feet up on highway pegs. Otherwise, even when rolling down the ramp of my bike storage, down the driveway, and finally to the street...I probably have tapped the foot lever and hand lever more times than I can remember.

As this conversation has continued, we have discovered there's old components, and contaminated/deteriorated brake fluid. The owner has installed new pads, and his dealer has replaced the fluid. In either of those actions, we have no way of knowing if there was any technique (too aggressive pushing the pistons off the rotor??), over filling the reservoir, etc., that could have caused this issue.

One thing I do know is that if this were my machine, I would lack confidence to risk it in traffic until I find the problem and solve it.

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post #43 of 46 Old Oct 17th, 2019, 9:08 pm
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Re: Brakes coming on uncommanded.

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Not as yet. My searches have not turned up anything compatible in the UK. Goodrich is the name for SS brake lines here, but they don't list a late LT. Speigler is an option if the import cost wasn't stratospheric!

I will replace them as soon as I can, they have been under suspicion from the start.
How about Galfer they are at least in the EU.

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post #44 of 46 Old Oct 18th, 2019, 9:09 am
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Re: Brakes coming on uncommanded.

Go back to the second post..........


A front brake line line developed a leak at the point where the rubber hose is shrouded by a metal cover near the caliper.
This happened accidently after I was working on the calipers. The metal sliced into the hose creating a small leak which I did not spot.
I rode the bike to test the brakes for a few miles (the calipers had been slightly sticking causing one caliper to operate more than the other and resulting in uneven wear).
The brakes seemed to work fine and then the red warning light came on. I thought this must just be a low fluid level but very quickly afterwards the bike started to slow down itself, as if the brakes were being applied.
When I stopped there was a load of fluid all over the front wheel and the fluid was empty. I did not apply the brakes to stop.....it seemed to happen automatically as the fluid leaked out.

I fitted new lines and started the bleeding process. Bleeding the pump requires many operations to get all the air out.
On one occasion after I released the brake lever the pump started again by itself. I turned off the ignition and the only fault I could find was that I had let the reservoir go very low. I topped it up and made sure it never went that low again and that was 3 years ago!

Next job is the crank seal and replace slipping clutch........if I am brave enough!!
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post #45 of 46 Old Oct 18th, 2019, 9:31 am Thread Starter
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Re: Brakes coming on uncommanded.


Interesting points in the comments about the rear brake locking up.

I have sent an email to Goodrich inthe UK about availability, so will just have to wait now. I believe that they will make them up if need be.

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post #46 of 46 Old Jan 13th, 2020, 4:23 am Thread Starter
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Re: Brakes coming on uncommanded.

To conclude this thread, the problem seems to have gone away.

I had a 48K service in October which I paid a lot of money for and requested "particular attention to the brakes". I then went to France, doing 450 miles and followed with a little commuting. The seizing issue was still there until I noticed that the front brake was making a lot of noise. Inspection showed that the right hand front pads were on the metal, so thanks for a "professional service with particular attention to the brakes!"

The right hand caliper seemed reluctant to retract but eventually did and fitted new pads. I would have liked a new rotor, but it wasn't bad enough to justify the cost. I did buy a spare caliper just in case. Once the brakes had been run in, I found that they no longer have the fault. I'm not sure why I had convinced myself that it was the rear that was at fault, except for the fact that they were the last item I touched.

I spoke to the mechanic who did the service and he assured me that he pads were fine when he "took them out for inspection". Shame really, it means that he gets no further work from me.

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