Annoying master cylinder.

[gdouaire]

Installing a 1989 Caprice master cylinder in a gBody. Got the proper booster unit from 1989 Caprice. This is a Napa UAP rebuilt unit. Everything is fine, except, when we press the brake pedal, there seems to be a "mechanical stop". When we gently punch the master cylinder with a rubber hammer, then the pedal travel is back to normal -going much more deeper /almost to the floor. And then mechanical stops occurs again once the pedal is released and depressed again.

When bleeding the system, the pedal will sometimes go to the floor and create a lot of fluid volume, sometimes not and almost no fluid out at the bleeder.

Tested the pedal travel without a master cylinder bolted in, and travel to the floor is OK.

Running and adjustable prop valve, but it's set to have brakes at all 4 brakes.

Anyone ever experienced this?

I'm PISSED.

 

[Bonnewagon]

I've never heard of a mechanical stop on a master cylinder. Did you bench bleed it by rigging some short brake lines that flow right back to the open reservoirs? If so and it still happens I would say the MC is defective. If it doesn't then I'd be suspicious of the proportioning valve.

 

[gdouaire]

the issue was the master cylinder. While bench bleeding, ai only got very limited pedal travel. Wasnt sure then if that was normal and did not want to force anything.

Decided to take the master cylinder apart, and then reassemble it. I now get a lot of travel. Something was stuck. Maybe from being for a long time on a shelf...



In other words, the smaller part of the cylinder was stuck and no matter how much pressure I put, only the larger part of the cylinder would move and compress the first spring. Did not get much pressure to the wheels.

Finding is: always take apart your rebuilt master cylinder before installing it.

 

[gdouaire]

maybe the seals have been installed inside out. these are tappered. TBC.

 

[pontiacgp]

is the seal was put in backwards that would not stop the piston depressing but it would prevent it from returning and locking the brakes on. I would never recommend to anyone to take a rebuilt master apart to check it, it is not hard to tear a seal installing the pistons. If there is a problem bench bleeding a rebuilt master then take it back and get another