AARGH!! Back brake help please!

[JohnJ]

This is weird.

I set off on a trip in the last of 2012s warm weather. 10 miles into the journey the back brake came on. On its own, I hadn't used it. It stopped the bike and stayed locked.

The caliper is a Gremeca twin piston one and the master cylinder is the standard one with the reservoir built in to it.

I pulled out the R clip and removed the rod to the master cylinder, no good. I then took out the pads, damaging them as they were clamped tight to the disc. And carried on my way.

Back at home I checked the caliper over, all free. Next the master cylinder which was fine as far as I could see. I blew through everything with the airline and didn't find anything wrong.

Went for a ride. Same thing again. It stopped me dead.

This time I waited for it to cool and it freed off. I rode home and it didn't do it again.

Checked everything again and noticed the braided line was deteriorating inside so I renewed it.

Went for a ride, it did it again! The rubber in the top of the reservoir had fluid on the top of it indicating a hole in it so I renewed it.

Strange thing is if I ride for 5 miles or so the disc is cold, but when it does its trick and jams the disc is red hot, it's as if it isn't a build up of something binding but a sudden decision to lock up.

Any ideas, it's driving me mad!

[ruffle]

Are you using the back brake?

Sounds like the pistons are sticking (not freely returning into the caliper body), which causes the pads to rub on the disk which overheats everything and eventually locks the brake.

Or the fluid return hole in the master cylinder is blocked?

Or the return spring in the master cylinder is borked?

Or you should have a proper bike with a drum rear brake

[JohnJ]

I hardly use the back brake, if I do it's usually to hold me at a junction.

The pistons seem free enough when the caliper is cold. I can blow through every hole in the master cylinder easily. When I left it to cool it freed itself.

Just going to strip the caliper again now.

[KeithZ1R]

Change the fluid !!, heat water steam expansion ,the rest is feedback physics .
unless its new fluid but I would change it anyway just an Idea

[zed1015]

A closed system is the cause.

The brake system should be open to the reservoir for fluid expansion and contraction when the brake is not being applied.

Check the reservoir cap for a blocked breather as this will create a sealed system and allow pressure to build up in it, thus activating the brake.

[z1bman]

i would say you have not got enough free play in the master cylinder adjuster rod. try disconnecting the rod from the lever + go for a ride to see if it still sticks on. or you could have the lever set too high + are activating the brake unknowingly i have seen this many times

[sid123]

what z1b man says - I ve also seen this many times


sid

[JohnJ]

what z1b man says - I ve also seen this many times


sid

No, it's not that, there's plenty of play. Previous to this happening the bike has done a good 10k miles with the same back brake setup. the rod is not adjustable and has play once the piston in the master cylinder is against its circlip.

[z1bman]

when this problem occurs have you still got free pay in the adjuster rod?
make sure you have not over filled the reservoir you must have an air gap between the fluid + the rubber seal. might be worth removing the master cylinder piston to make sure everything is clean + working smoothly

[JohnJ]

when this problem occurs have you still got free pay in the adjuster rod?
make sure you have not over filled the reservoir you must have an air gap between the fluid + the rubber seal. might be worth removing the master cylinder piston to make sure everything is clean + working smoothly

Yeah there's still play in the rod when it happens. I've had the master cylinder to bits and all works smoothly.

The reservoir isn't too full either.

I'm baffled.

[gray]

seen this once before when a rubber brake hose had collapsed internally , making a 1-way valve. Looked ok from the outside , but fluid could only travel "to" the caliper, and not "from" it .

[z1bman]

i would say you have not got enough free play in the master cylinder adjuster rod. try disconnecting the rod from the lever + go for a ride to see if it still sticks on. or you could have the lever set too high + are activating the brake unknowingly i have seen this many times

try this + if it still has the problem the fault is in the calliper or the brake line. is anything interfering with the brake pedal like exhaust or anything rubbing against the calliper

[jphaynes669]

If all the mechanical components are free moving then it has to be a hydraulic problem.
A fluid system has to be a able to breath, if it cant it will build pressure. Something is creating a non return valve situation.The brake lock is being brought about by the pads binding a little more than needed, heat expansion sets in and it locks the wheel.
Strip the whole system again, replace all rubber components seals etc , new fresh fluid,
I presume the system bleeds up ok, try running the system with the fluid on the lower level and with the resevoir top off. You could do this with the bike on its centre stand or jacked up, running the bike in gear operating the brake. If you have a infa red laser temp gauge you will be able to record disc temperature in stages.
Make sure the brake disc is in good condition, within thickness tolerance and is not warped.
Good luck and let us know the results.