rear brake master cylinder problem

[peter250r]

g'day people,

the problem is that i think that my rear master cylinder on my 86 250r may be leaking.

for a long time the rear brakes have not worked, just recently i took apart the rear master cylinder (since the rear caliper was just reconditioned) and that was a major job in itself. the piston was seized in there so i had to drill it out. in doing so may have scratched or scored the cylinder in which the piston moves.

after putting it back together with the new piston and seals etc i find that its not building pressure. and when i press the brake lever down and release it i can see the brake fluid going down the hole of the reservoir and coming back up again.

so my conclusion after seeing this is that the master cylinder needs replacing. does anyone else have the same conclusion?

[wonderboy]

If you see fluid moving up top with no result at the caliper, then you have air in the system. Bleeding can be a pain sometimes, but you have to get every little bit of air out of the system. Air is compressible and will compress instead of making your caliper pistons move.

Have you tried a thorough bleed?

[poohbee1]

I would bleed it more also but you might as well rebuild the master cylinder .I have learned from the front brakes on mine rubber doesn't last 30 years in brake fluid .

[peter250r]

yep i tried bleeding it through until there was no more air coming out the bleeder valve. but still there was no pressure on the brake pedal.

the other day i got my dad ,whose a mechanic,(i hadn't really had a good look at it yet) to have a look at the inside of the master cylinder.

he reckoned its no good, too deeply scratched from drilling the old piston out.

he said hes got a mate in America that can get one sent over cheaper than if i got one off ebay that was in America.

[Flyingw]

Hey Peter, before you give up on it completely, try bleeding it at the banjo bolts instead of the bleed valve at the caliper. Air bubbles have a real bad habit of hiding and are stubborn in coming out. Bleed at the top banjo bolt first then the bottom one.