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Discussion Starter #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.
 

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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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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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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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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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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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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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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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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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Discussion Starter #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.
 

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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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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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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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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... :grin: ).... 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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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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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.
 

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Discussion Starter #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.
 

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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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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:
 

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Discussion Starter #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?
 
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