The 100-Six Forum

Hello all -- I have a BN6 with a 5/8" bore brake master cylinder, and am not having any luck with the rebuild kit I received from Moss. I'm wondering whether anyone else has had the same issues, and am beginning to think that the master cylinder (or even just the piston) I have is aftermarket. The piston seal OD seems to be correct, but the ID and thickness seem wrong (too small ID and to too thick to fully fit into the groove in the piston -- the supplied seal is very difficult to get fitted in place, and there's no way to get the piston into the bore once it's fitted.

I may see if I can track down an original piston (assuming maybe mine is aftermarket) and see whether the rebuild kit seal fits on it. I'm baffled at this point though.

For what it's worth, the master cylinder I have is marked "5/8" on the top (between the inlet and outlet), and has a faint stamp hear the mounting flange that looks like "DIALOY" (or something like that). Not a Girling part, apparently. The piston has the main seal and a second dust seal further out. It also has a third groove that's not used but looks like it could accommodate an o-ring.

Any input would be appreciated. Thanks!

This is just off the top of my head so you may want to confirm this first. But, the disc brake 3000s used 5/8" master cylinder bores. I am pretty sure that the 100/6s with drum brakes used the larger 3/4" master cylinders. If you ordered a 100/6 - BN6 rebuild kit then it would be the wrong size - too big for your 5/8" bore.

It's definitely a 5/8" MC -- the outer diameter of the seals is correct, but the inner diameter is too small for the diameter of the ridge in the plunger. Frustrating!

Thanks for the reply though.

Probably best off buying a new master cylinder.

Already done and installed. I just have a rebuild kit and a perfectly good old MC that I'd love to be a able to use too; a simple plunger would make that possible. The search continues!

Pegasus has Girling repair kits in all the sizes:

https://www.pegasusautoracing.com/productselection.asp?Product=3504

Thanks -- I think the issue with those will be the same as the issue I have with the current rebuild kit. OD of the seal is fine, but the ID is off, and it won't fit around the plunger (hence my need for an original plunger). I'll look closer at the pegasus offerings though; thanks again.

If it's not a Girling, some of the Moss cheapos have been non-rebuildable.