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Brakes coming on uncommanded.

3K views 45 replies 14 participants last post by  Scimitar 
#1 ·
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.
 
#2 ·
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.
 
#3 ·
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.
 
#4 ·
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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#5 ·
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.
 
#6 ·
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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#8 ·
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.
 
#7 ·
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.
 
#10 ·
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.
 
#11 ·
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…..
 
#13 ·
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.
 
#15 ·
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.
 
#16 ·
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.

Ride safe.
 
#17 ·
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.
 
#18 ·
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.
 
#19 ·
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????:serious:
 
#20 ·
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?
 
#21 ·
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?
 
#24 ·
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.:surprise:
 
#25 · (Edited)
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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#26 ·
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.
 
#27 ·
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.
 
#28 ·
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.
 
#33 ·
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?).
 
#34 ·
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.
 
#35 · (Edited)
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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#38 ·
Have you replaced those brake lines yet? If you are running the originals, 15 years is a long time for the OEM hoses.
 
#44 ·
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!!
 
#45 ·


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.
 
#46 ·
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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