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Sticky Master Cylinder piston

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nas90tdi

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I just rebuilt my 7/8" MC with a Moss rebuild kit. Bores are in great shape. No grooves or ridges. I honed and polished the cylinders. Clutch side works flawless.
After I rebuilt I noticed the brake side piston seemed a little sticky. Put a little more fluid on both sides of it for lube and ran it back and forth several times and it seemed better. It sat for a few days before I got a chance to try to reinstall. Ran it back and forth to check and the brake side stuck again. I took it apart to make sure I hadn't missed something. It all looked good. Put it together and it was a little tight but not bad. Installed on car, bled brakes. Now the brakes work great. ONCE. Then the cylinder hangs for a bit and then eases back to fully open.

I know I have seen a thread about this on some board, but haven't been able to find it. Any thoughts on this? Think it will work itself loose with a little use?

Thanks,
Kris
 
What type fluid are you using? How is the "stop" bolt adjusted?
 
Dot4. It's not a stop bolt issue. The physical piston of the master cylinder is hanging after compression, then slowly coming back up. It's almost like the new cups and seals are slightly too big. Physical comparison matched the old ones I removed. But that's just visual, so not really any indicator of the true size of the new seals.
 
I dunno. :(
 
Was there a solution for this other than a new master? I've rebuilt mine with the Moss kit (7/8) and the seals are extremely tight in the bores. If I push the pistons back with a rod, the springs will not return them even after sitting overnight. There is fluid in the reservoir. It's not yet on the car. I really don't want to install it and have it not work.

This is just one more example of aftermarket parts that look OK in the box but don't fit. Examples: tie rod ends had to be shortened, threaded ends shortened; front brake cylinder bodies ground to clear shoes; rear brake shoe diameter too large for drums; supplied shims too thin for king pins. Biggest surprise was that the replacement front bearings were perfect; I was expecting problems there.

Jim
 
Do you have disc brakes? If so, you may have a 3/4" bore master and the 7/8" kit will be Very sticky!
Rut
 
nas90tdi said:
I just rebuilt my 7/8" MC with a Moss rebuild kit. Bores are in great shape. No grooves or ridges. I honed and polished the cylinders. Clutch side works flawless.
After I rebuilt I noticed the brake side piston seemed a little sticky. Put a little more fluid on both sides of it for lube and ran it back and forth several times and it seemed better. It sat for a few days before I got a chance to try to reinstall. Ran it back and forth to check and the brake side stuck again. I took it apart to make sure I hadn't missed something. It all looked good. Put it together and it was a little tight but not bad. Installed on car, bled brakes. Now the brakes work great. ONCE. Then the cylinder hangs for a bit and then eases back to fully open.

I know I have seen a thread about this on some board, but haven't been able to find it. Any thoughts on this? Think it will work itself loose with a little use?

Thanks,
Kris
Click to expand...


Hi Kris,

Potentially, it's a slightly oversize seal. By that I mean outside the tolerance range. The other thing to check is that the seal is properly seated. There is a groove in the seal that must seat firmly over the lip in the piston. If it's not fully seated, it will expand the seal and cause the problem you describe.

One of my specialties is a conversion rebuilding/sleeving of these tandem master cylinders for use with disc brakes, but I can also brass sleeve standard or combination size bores. Typically, people find a 3/4" bore for the clutch works better than the original 7/8", while keeping 7/8" bore for drum drakes. It's not only cheaper than buying a new one, but you will be keeping an original part rather than a sometimes questionable reproduction, and in addition, the brass sleeved ones have a much longer life.

See me web site for more info on these master cylinders.

https://gerardsgarage.com/Garage/Tech/DbrakeUg.htm
 
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