Intriguing brake fault, BMW bike

[ajlelectronics]

Three times now in as many weeks, the rear brake has decided that it will deploy, bringing me to a rapid halt. This despite the brake not having been deliberately used just before. It is a BMW K1200LT ABS which has servo assistance and linked braking. It is definitely the rear brake, the rotor could have fried an egg it was that hot.
To release it, pumping the brakes made no difference, but cycling the ignition released it.
About 3 months ago, I replaced the brake pads on the back with some OEM equivalent quality brand and had no problem until recently.

My thinking is that there is nothing that can do that, anything in the brake lines like the ABS can only defeat pressure, not apply it. The next idea is degraded brake hoses acting like a one way valve. However that surely should mean that the brakes wouldn't release after operation. In my case the brakes had happily come off and self activated a little later.

 

[cocker]

Brake fluid level ok ?
I don't know about that abs stuff , but I assume you still need to check levels after you've jammed the pistons back to get the new pads in .

 

[merryman]

Had brakes come on once on my drum braked MG, turned out to be mater cylinder sticking so fluid couldn't return to reservoir and brakes were still slightly on, so they got hot and the fluid expanded, putting them on more and so getting them hotter, until I could feel them pulling. You would have thought the pressure would have shoved the master cyl back out, but it didn't.

 

[hotponyshoes]

I don't know anything about bikes but I would not expect a rotor to get hot from just one stop.
I would guess there is some brake binding on the rear, as it gets hotter the fluid in the piston is expanding and pushing the brake on more until it hits boiling point then the rapid expansion is hitting the brake on.
Jack up the back and see how freely the wheel spins. Take for a short run without using the back brake and Jack it up again and see if there is any heat or binding.

 

[Rannsachair]

I hate BMW linked ABS servo systems, I had a frightening failure going down The Rest and be Thankful on my R1100S

I would check the rear caliper pistons are free, a brake flush can also help if the fault is at the abs block. I assume there is no warning light coming on?

 

[Fazerruss]

As fas a I know the brake system on that bike can apply the brakes itself. My brother used to have the same one and it has a system which applies the brakes when on side stand if the bike starts to roll forward so preventing it from toppling over.
You could potentially have a problem with the electronics but I would start with the basics first and strip the rear caliper. If it binds it will cause the disk to get hot and jam on like you mentioned.

 

[hotponyshoes]

Again, I only know about cars but abs can apply pressure quite happily.
Stuff with emergency brake assist will apply more pressure than the driver can with the foot pedal!

 

[Rannsachair]

If you put the bike on the centre stand with the ignition off and brake hard on the pedal a few times - does the rear brake stick on? If so likely to be the caliper or as you say a collapsed hose.

If that is ok try the same with the ignition on - may give an indication if it is an abs valve issue if it only sticks with the ignition on, though with the servo system will apply the brake with a lot more force than just manually.

 

[pedrobedro]

The next idea is degraded brake hoses acting like a one way valve.

Brake hoses are how old ? I've seen hoses with cracks on the outside and never seen one break down internally.

 

[earthman]

As fas a I know the brake system on that bike can apply the brakes itself. My brother used to have the same one and it has a system which applies the brakes when on side stand if the bike starts to roll forward so preventing it from toppling over.

Oh wow, I've never heard of that before,.....if the brake applied every time the side stand were used, I can kind of understand that, BMW invented an 'handbrake' for a bike but to wait until it actually starts to roll forward??

 

[Fazerruss]

Oh wow, I've never heard of that before,.....if the brake applied every time the side stand were used, I can kind of understand that, BMW invented an 'handbrake' for a bike but to wait until it actually starts to roll forward??

Yes, my brother actually watched it do it. He was about to run for it but it stopped itself.

 

[Craig-SM]

Brake hoses are how old ? I've seen hoses with cracks on the outside and never seen one break down internally.

^This and also check the pads are moving freely when released as the heat build up expands the disc making it bind until it cools.

 

[premmington]

Three times now in as many weeks, the rear brake has decided that it will deploy, bringing me to a rapid halt. This despite the brake not having been deliberately used just before. It is a BMW K1200LT ABS which has servo assistance and linked braking. It is definitely the rear brake, the rotor could have fried an egg it was that hot.
To release it, pumping the brakes made no difference, but cycling the ignition released it.
About 3 months ago, I replaced the brake pads on the back with some OEM equivalent quality brand and had no problem until recently.

My thinking is that there is nothing that can do that, anything in the brake lines like the ABS can only defeat pressure, not apply it. The next idea is degraded brake hoses acting like a one way valve. However that surely should mean that the brakes wouldn't release after operation. In my case the brakes had happily come off and self activated a little later.

If you were closer to Norfolk - your could have plugged it into my GS-911 diagnostics - these K series have auto "hand brakes" that play up (as Fazerruss has already mentioned).

My first port of call would be re bleeding it - if the hoses are collapsing - you will see if the fluid runs free - when you bleed it.

All bikes will have joined brakes and ABS into the future - oh what fun!

 

[atomant48]

If you weren’t on the brakes just prior then I would be suspecting the electronics over a hydraulic issue. Calliper can’t stick in under hydraulics if it hasn’t been applied in the first place.

My first GS had a couple issues with the linked brakes - it had a habit of locking up the rear wheel if you applied any rear brake at the same tine as the lever. I guess the ABS wasn’t coping properly. My adventure seems a lot better but I guess there have been a bunch of software pumps over the years.

I bet it’s a bit unsettling though - apart from the handling issues for you I’m not sure the final drive will appreciate a sudden “under rotation” either.

 

[ajlelectronics]

Brake fluid level ok ?
I don't know about that abs stuff , but I assume you still need to check levels after you've jammed the pistons back to get the new pads in .

I didn't check brake fluid because my assumption was that with the pistons pushed back, the level wasn't going to be lacking! I will double check though.

 

[ajlelectronics]

Take for a short run without using the back brake and Jack it up again and see if there is any heat or binding.

Nice idea but not possible! It has linked brakes. Pull the front and the rear also deploys. Same deal when using the rear brake, the front deploys.

 

[ajlelectronics]

I hate BMW linked ABS servo systems, I had a frightening failure going down The Rest and be Thankful on my R1100S

I would check the rear caliper pistons are free, a brake flush can also help if the fault is at the abs block. I assume there is no warning light coming on?

No, no warning lights. I will make some space to get it in the workshop this week. It suggests that it is connected with the pad change, or it is one hell of a co-incidence.

 

[ajlelectronics]

As fas a I know the brake system on that bike can apply the brakes itself. My brother used to have the same one and it has a system which applies the brakes when on side stand if the bike starts to roll forward so preventing it from toppling over.

Nobody seems to be aware of that on the BMW forum! Not sure how it can work with the engine / ignition off either because the servo isn't going to work.

 

[ajlelectronics]

Brake hoses are how old ? I've seen hoses with cracks on the outside and never seen one break down internally.

Factory originals 2005. I have had cars through the workshop where the brakes wouldn't release. That was down to internal degradation of the flexis causing a one way valve effect. Full cure, change the flexis all round. I would have done the bike ones, but I can't find any! Goodridge don't seem to do the kit and Speigler in America will cost a small mortgage.

 

[hotponyshoes]

Pad change might not be a coincidence ,
If there was some corrosion inside the caliper it might have been behind the piston as the piston would have been sticking out further due to the pads being thinner.
As you will have pushed in the piston to fit the new thicker pads the piston might now be sticking on the corroded part.

Might not actually be corrosion, just a bit of gunk if the fluid has not been changed regularly or damp has got in there.