Mark IV rear master cylinder shot?

[Bazil]

The linked brakes on my Mark IV have been "sub-optimal" for a while, which I attributed to a the need of a bleed. I've replaced brakes lines and pads recently and brakes have worked, but not as well as I would expect ( front lever is great though)

Foot lever would extend until it hit the exhaust without much effect, pumping didn't make much difference. Took the caliper off the bike to make bleeding easier, but found virtually no fluid being pumped out once the bleed nipple was cracked. Pads wouldn't close up. I pulled the calliper apart - no sign of leaks and the pistons came out with a satisfying pop with some compressed air. No sign of anything else untoward in the caliper. Cleaned, re-assembled and re-installed - now scratching my head.

Tried reverse bleeding as described on WG but couldn't get any fluid back into the reservoir ( did succeed in spraying brake fluid over the garage floor though!)

There's no evidence of leaking fluid anywhere, except of course on the garage floor.

I'm guessing that the rear master cylinder is the cuplrit and needs a re-build - what's the view from the collective wisdom who gathers here? Is there anything else I need to look for ( blocked manifold for instance?). Or just go and buy a re-build kit?

cheers

Bazil

[rodekyll]

I wouldn't have real high confidence in the m/c being the culprit, but it costs little to rebuild it and probably can't hurt anything to do the service.

[Curtis Harper]

http://www.harpermoto.com/rear-master-cylinder-kit.html

In stock and can go in tomorrow's post

[lucian]

Check and make sure the the linkage hasn't been adjusted up beyond letting the master cyl. return all the way out. Fluid can't get to the supply port on the cylinder.

[Bazil]

Thanks Lucian - I'll check that out. Wouldn't have occurred to me that the linkage needed to have more freeplay, but I did try adjusting the linkage tighter in case the mc actuating arm wasn't throwing enough - never thought about it not being able to return enough.
If thats unsuccessful, I guess I'll order the rebuild kit.

[lucian]

That very well could be it. My daughters v50 w/ linked brakes came with no bake action whatsoever as it had sat for years. I couldn't bleed out the rear to save my ass until I discovered the link rod had been fully extended by whoever messed with it last. I cranked it back in so the piston could return all the way back and it pumped right up. Hope it's that simple. I think I bled the front caliper first and then the rear if I remember correctly . good luck !

[Tobit]

I assume that the MC piston is returning to it's normal position?  A common mistake is to pump the pedal all the way down while bleeding.  The piston never travels that far during normal use and if the fluid hasn't been changed in a while sludge can build up in the MC bore that isn't normally stroked.  The piston will hang up deep in the bore.

Been there done that. 

As you can't reverse bleed, it could be that the MC piston is stuck, blocking off the reservoir.

Just guessing.

Tobit.

[Chuck in Indiana]

I assume that the MC piston is returning to it's normal position?  A common mistake is to pump the pedal all the way down while bleeding.  The piston never travels that far during normal use and if the fluid hasn't been changed in a while sludge can build up in the MC bore that isn't normally stroked.  The piston will hang up deep in the bore.

Been there done that. 

As you can't reverse bleed, it could be that the MC piston is stuck, blocking off the reservoir.

Just guessing.

Tobit.

Yep. I did a tutorial on rebuilding the linked brake master cylinder on the Lario Rehab thread..