[dstryr]

I'm having trouble with master cylinders on my 69 PDB GTO. Anyone else having trouble with them not holding up? I bench bleed and install them and after several drives the brakes are soft again and the pedal goes to the floor. I only drive it a few times a year and mostly bc the MCs keep failing. I have gravity bled the lines, used a mighty vac and when I'm done the brakes are good.

I think I've put 4 on in the last 7-8 years. 3 of the MCs have been autozone or equal. The last one is came from PY last summer and bought it because it looks correct under the hood and I figured for $80 it might last a while. Was driving a few weekends back, 25 miles from home the pedal felt soft. By the time I got home they were pretty much gone. Popped the cap off, reservoirs are full and the pedal goes to the floor.

Am I missing something? Help!

[Ram Air IV Jack]

I rebuilt my original years ago and never had an issue. I do know that the Chinese knockoffs are not very good and that may be part of your problem. Someone here I'm sure will be able to comment on that. So much of this stuff is made overseas now that it may take several tries to get a decent part. You read about this all the time. I'm wondering if the brake fluid is affecting the seals on the piston in the mc? Fluid would just push past the seals, and you would have a soft/no pedal situation as you've mentioned.

[Scarebird]

Avoid Cardone and Autozone - they are strict price point vendors.

Centric and Raybestos do a good job as does Bendix.

When you do get one, take it apart and wet down the seals and bore to make sure they do not tear. New is best here - they are still being made by AC Delco.

https://www.rockauto.com/en/catalog/...+cylinder,1836


New is best here - they are still being made by AC Delco.

https://www.rockauto.com/en/moreinfo...pt=1836&jsn=19

[ponchonlefty]

it seems the rubber products have a deterioration problem. i dont know what changed but that could be what is going on with the mastercylinders. or am i wrong?

[dataway]

Even on new MCs I pull them apart, clean them, hone the bore and reassemble. I use old school type hone with the straight stones, not a ball hone. Can make a huge difference in how well they seal.

[dstryr]

Thanks everyone for the info, and Scarebird, appreciate the links to rock auto. I'll get a hone and rebuild kits and should be able to salvage them.

[dmorg1]

It is possible that you have a master cylinder that has a seal installed like this one.

If I recall correctly this MC was purchased from MP Brakes.

This was a brand new out of the box MC that I pulled apart in order to machine for a residual valve.

Obviously this seal was installed in a very poor fashion.

It's unlikely that this is the only out there with an ill fitting seal.

Dataway, that is sound advice. Pull em apart, confirm and correct.


https://forums.maxperformanceinc.com...1&d=1657757600