XT600 3AJ Rear Master Cylinder Alternatives
 

Hi, does anyone know if there alternatives from newer bikes for XT600 3AJ master cylinder.
Planning trip in a week's time and have rear brake problems. Wondering if cylinder bore is worn.

What's the exact problem, and have you tried changing fluid and careful bleed?

Nope wont be worn, possible new seal on the piston but its not leaking correct? so its not your master cylinder.

Service your caliper (sticking/tight piston?) fresh fluid & bleed it properly.

Mezo.

Hi Grant, Mezo

Fitted new seals to caliper and master cylinder. Bled the brakes (normal method) but very long travel on brake pedal, and just slows down bike a little.

Next thought was to to bleed brakes by reverse method (very small syringe hopeless), and was going to put two thin pieces of plastic between brake pads and pump them together, then fill through nipple. However, I could not pump the brake pads together fully.

Prised them apart and (in order to check the seals were fitted properly) tried to pump piston out, but piston now stuck most of way out (and I cant force it back in). I will take off the dust seal and then try again with a newly filled system.

Maybe seals restricting piston movement?

Massive syringe arrives today which should make reverse bleeding easier and cleaner.

I hope that you are both correct, and it is just air in the system.

My thoughts about the bore in the master cylinder being worn was that, with the new seals on, the piston fitted very easily into the bore. How does it manage to generate the force with such an easy fit?

Anyhow, going away with my 2 sons in a weeks time....Got to fix it.

Thanks for your help. :thumbup1:

m/c seals are very soft, they start sealing as soon as there is minimal pressure behind.



most likely there are still bubbles in there.

Never would have thought this was the problem!
 

I finally sorted out the poor rear braking. It wasn't the hydraulics. I stripped the caliper twice, checking seals and piston and reverse bled. I was confident there was no air in the system.

It turned out to be the brake pedal. The mounting hole was worn. Oval rather than round. This was causing excessive play in the whole assembly. So when pushing the pedal it was moving sideways, as well as down. This meant less effective movement on the piston. Only a few mm, but enough to make a significant difference.

Shimming the pedal to remove sideways movement sorted the problem.

Glad you got it sorted & good of you to give us the feedback it all helps for future diagnosis.

Mezo. beer