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Master Cylinder OR Proportioning Valve
#1414147 Thu Jun 17 2021 04:20 PM
Joined: Mar 2012
Posts: 37
Pete O Offline OP
I have a 55 second series truck with a 1978 Camaro front suspension. Disk brakes in front and drum breaks in the rear. The master cylinder leaks from the cover. I have replaced the gasket and it still leaks. Could it be the proportioning valve? The breaks work fine.

Re: Master Cylinder OR Proportioning Valve
Pete O #1414153 Thu Jun 17 2021 06:06 PM
Joined: Feb 2004
Posts: 22,587
Don't overfill the reservoir. A little air gap below the gasket is called "expansion space"- - - -particularly on a non-vented system.

The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
Re: Master Cylinder OR Proportioning Valve
Pete O #1414475 Mon Jun 21 2021 09:52 PM
Joined: Dec 2017
Posts: 954
Might be. Those proportioning valves have a shuttle cock inside that acts as a trigger for the brake light sensor. It also stops fluid flow if there is a failure in the brakes, say a rear wheel cylinder fails, the loss of pressure forces the valve to close as to the rear brakes. That may be the fluid out of the cap or it could just be as HRL noted, that you are overfilling the master.

Just as an fyi on the proportioning valve, one of the issues is that to bleed the brakes, you actually have to remove the brake light switch and insert a small tool that keeps the brass shuttle cock centered. otherwise when you crack open a cylinder or caliper, the loss of pressure moves the shuttle cock and you have to then get it re centered. I just went through this same thing with a ford truck that had essentially a chevy brake system on it. We discovered the rear brakes were not working and the shoes were in such good condition it looked like they had never worked, although the owner, the uncle of a friend of mine, swore up and down the truck stopped fine. It ended up the valve had tripped and was in the tripped position so long it was now simply corroded in place. The rear brakes had not in fact been working at all. Replaced the valve and properly bled the brakes and the truck stopped like is should.

Re: Master Cylinder OR Proportioning Valve
Pete O #1414486 Tue Jun 22 2021 12:14 AM
Joined: Mar 2004
Posts: 5,126
Great topic!

Will a continuity tester across the switch confirm if the shuttle is centered or not?

If it's not centered how do you re-center it?


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