dead master cylinder

[Glyn]

can some one comfirm that my rear brake master cylinder is dead

after riding bike home noticed the power felt down quick check revealed rear brake binding
took caliper off the bike and removed pads, every thing looked good
the pistons moved in and out o.k but the fluid wont go back to the master it just pushes the other piston out.
the only way to get both pistons to return was to loosen of bleed nipple. they both moved home freely then.
so it looks like the fluid cant return to the master
its not an over adjustment of the pedal as i removed it just to check.

so is the master cylinder servicable
if not any body got one

[Jools]

Don't think it's dead mate...I've just had the exact same thing where my rear brake was sticking on.

I put it down to the fact that I only use the back brake once a year when I put the bike through it's MOT, I had just bled the brake to make it work well enough for that. I guessed the pistons were crudded up, through lack of use so I took the calliper off, whipped the pads out and gave it all a dose of WD40 to clean it up (obviously not the pads or the disc) and greased the pistons up with brake grease.

But mine was the same, the pistons wouldn't budge until I loosened the bleed nipple, so I just let the brake fluid come out until the pistons were back in place, then reassembled it all and bled it through again....sorted.

[Glyn]

thanks for info jools
ive already tried new fluid new pads and a good clean up.
and as i say the pistons return no prob with the nipple loosend
but the brake is still binding:(
might just remove the whole thing:devil:

[Derek]

Check that there is freeplay at the rod linking the pedal to the master cylinder . If there is no freeplay it can prevent the piston from returning fully. It needs to fully return to uncover the passage to the reservoir. If this isn't happening the the caliper pistions won't go back and brake will stick on.

[Glyn]

thanks derek
it was the first thing i checked as my mate (you know who you are) burnt a couple of rear calipers out after adjustment of free play

the only thing i havn't tried is stripping down the master cylinder
thats the next step.