Brake issues....stumped!

[pontiacgp]

I didn't have to push the pin in, it went back in on it's own when I was slamming on the brakes going in reverse. I use speed bleeders and nothing was coming out the back so I was just trying anything to get the valve back in position.


I found this online....I guess what I did equalised the pressure

The blocked side will trickle fluid out when the bleeder screw is cracked and the pedal pressed. An unblocked line will squirt fluid out the bleeder.
The lines that are clear must be left open and the blocked lines should have the bleeder screws tight to cause pressure to build up on that side. Be sure to use the standard bleeding procedures to prevent air from entering the system.
Slowly press the pedal with steady pressure a number of times until the light goes out; this will center the differential valve. You may also hear a pop come from the proportioning valve. This is the metering valve returning to its equalized position. When the light goes out, close the bleeder screw.

 

[Bonnewagon]

I think what Steve is describing is what happens when one side leaks and the proportioning valve gets moved so far out of whack as to actuate the "brake" light on the dash. That would be the single wire that goes to the combination valve and what it means is that one side of the system has failed. This is also why we have a dual chamber master cylinder for safety. He is correct in the procedure for resetting it back to it's normal position. But what Bill described is the little rubber covered button on the end of the combination valve that is supposed to be depressed in order to bleed the rear brakes. That is because the brakes are biased towards the front since under braking conditions the weight of the car tilts forward and the front brakes need the most pressure/volume. Don't forget that our cars use a combination valve that incorporates a proportioning/limiting valve to first take up the slack in the rear brakes so they actuate in concert with the fronts, and then limit their action, and allows more pressure/volume to the fronts because they do the heavy lifting. They also have a residual pressure valve to keep about 10psi in the rear drum brake lines.As I said before I have had just one car in my life that the button actually worked as it should and needed to be depressed to bleed the rears. All the rest bled just fine so I'm guessing the valve is stuck internally. And since I also don't experience any uneven brake wear between the fronts and rears I'm thinking it's not a problem.

 

[bill]

I never had a light on issue....so I can assume that the valve wasnt stuck in one position. Im just gonna let it ride for now...since it is 100% better than it was. Thanks for the info....

 

[pontiacgp]

if you have a lazer temp gauge it's an easy way to check the brake drum temp to see if the brakes are doing their job

 

[Bonnewagon]

And the best way to test the proportioning valve is to panic stop on wet pavement. If the rear end swings out, you need more rear bias. An adjustable valve can be used to do this, and I'm sure Steve is well versed on brake adjustments for the race car. Luckily the GM engineers set our cars up to brake well in all conditions. But if you see crazy handling or lopsided wear then there might be an issue.

 

[bill]

Ive tried a couple of panic stops and had no side pull or lock up. Wasnt wet though. I did get a slight rear lock up once but I think it was due to gravel on the road....cause it only did it once. I dont have a laser temp probe...although I wish I did...love those things.

 

[pontiacgp]

if you back up and slam the brakes on you'll know if the back brakes are working. The front of the car will raise up and if not the front brakes will just drag the car to a stop

 

[bill]

Tried your back up idea...and it stops hard...so the rears are working as they should.

 

[Bonnewagon]

Which proves what I said. Even though you can't push in the button, the braking action is unaffected. I still can't find an explanation for why it's used, maybe it releases the residual pressure, I just don't know.

 

[gbondo57]

I had a 90 GMC that where the vacuum hose plugged into the manifold, just below the tbi on the right. While running, I pulled the that line off, it changed nothing. I got a small screwdriver, after unscrewing the vac fitting, and dug out a nasty carbon build up that was plugging the vac supply. Solved it.