Back brake sticking. Help!

[dubber68]

OK so I have an issue with my rear brake cylinder/piston sticking. I have taken the calliper off and cleaned it, put new disc and pads as well as a calliper service kit. This included spring plate and retaining pin. I have bled the brakes and as soon as you apply them the piston does not retract enough to clear the disc. I have checked the fluid level and taken some out so that it is mid way between min and max. I can only think that the master cylinder is knackered. Your thoughts would be appreciated. I also thought it may be due to bleeding it without actuating the ABS. I feel a visit to the dealers coming up. By the way it is a BMW GS650.

[hotbulb]

Did the caliper service include replacing the piston seal? That generally acts as the "retraction spring". Plenty of rubber grease on seals and dust excluder?
Otherwise, could be a master cylinder fault... perhaps adjustment (pushrod not returning fully so not opening the recuperation port). Is the brake hose OK? I've had problems in the past where the inner lining starts to disintegrate and acts as a sort of one-way valve.
Or it's an ABS problem ... a complete mystery to me

[dubber68]

Must admit I didn't change the seal just fully extended the piston and cleaned and greased it. Might be the next thing to try.

[Tonibe63]

Is the caliper sliding ok on the mounting bracket pin?
Was the problem there before replacing disc and pads?

[dubber68]

The problem was there before changing disc etc. It seems to slide easily on the pin and the pin is brand new. Might need to put a day to one side and order a seal kit. Then totally disassemble and rebuild the caliper. Can't shake the feeling it's master cylinder related though but I don't know how to check.

[Tonibe63]

Corrosion under the caliper piston seal causing it to grip the piston would be my first place to look.

[zimtim]

http://adventurebikerider.com/forum/27- ... tml#511498

[Mad Hatter]

I had a similar thing the other week on the gf's bike after some tinkering with the back brake.

On hers on the master cylinder you can adjust the stroke from the pedal, its basically a bit of all thread from the bottom of the master cylinder, anyway, what I'd done is adjust it too harshly so it was actually permanently stuck on, seemed fine on a static test but as soon as it was tried on the run it locked on, lots of smoke and whatnot slightly worrying for me being behind her :sick:

Anyway, I've used the adjusters before when pads have been a bit low and not biting as much as they should, if someone's done that for whatever reason and you've stuck new pads in that might be worth a look (thumbs)

[Mike54]

Must admit I didn't change the seal just fully extended the piston and cleaned and greased it.

theres your problem

[dubber68]

Thanks for all the replies I have just ordered a new seal kit and to be safe a new master cylinder which I will fit if the seal does not work.