Ever see an adjustable valve in just the rear brake line?

[Bonnewagon]

So this 1978 Malibu I have been beating my head against the wall with, has no combination valve. Instead, there is some sort of an adjustable valve in JUST the rear line. The brakes totally suck. It takes two feet on the pedal to stop, and the pedal travel is almost to the floor. Brakes have been bled, drums adjusted, new shoes and pads. As soon as I saw this I asked where T F the combination valve was. For one thing, there is no residual pressure valve to take up some of the rear drum expansion. There goes your pedal travel. There is no proportioning valve to balance the front to rear bias. There is no limiting valve to limit the rear pressure. In short- no combination valve. There is a simple splitter for the front discs under the MC.

We plan on fixing this once the engine is sorted out. I am just wondering what the F the logic would have been to plumb the brake lines this way.

 

[Built6spdMCSS]

I've seen them on cars that have been ran at the track a bunch, shut off the rear brakes so you don't glaze them doing the burnout.

 

[spidereyes455]

Kind of what I was thinking. A cheap easy way to do burnouts by reducing or closing off the pressure to the rear brakes

 

[bracketchev1221]

Basically a restriction to the rear brakes to prevent them from locking up in braking. Since the car nose dives in braking unloading the rear they would normally lock up on a 50/50 system.

 

[Bonnewagon]

So how would you adjust this for a street car? Right now the pedal travel is almost to the floor, and it takes two feet to get the car to stop. I had a friend with a 1968 Firebird. He used an adjustable proportioning valve, but it used both the front and rear lines. We set it up by waiting for a rainy day, in a BIG parking lot, and intentionally locking up the brakes. The rears would usually skid first. When the bias was right, we locked it in.

 

[Built6spdMCSS]

You have a picture of what it looks like? Sounds like it's set to closed.

 

[Bonnewagon]

Next time I will try to get a pic. It functionally looks like this, with only a single in and out port. But the one on this car is probably 30 years old or more so it is bigger and clunkier.

 

[Built6spdMCSS]

That's what I figured it looked like. Tried turning it and see if it moved?

 

[565bbchevy]

Next time I will try to get a pic. It functionally looks like this, with only a single in and out port. But the one on this car is probably 30 years old or more so it is bigger and clunkier.View attachment 237078

I have that one installed in my Monte's rear brake line just before the rear wheel, I have a disc/ drum proportioning in the stock location but I have Wilwood rear disc brakes in the back and I used that to dial in the bias with rears so I can take more advantage of the sticky tires versus the frontrunners

 

[86 Salon]

It's an adjustable proportioning valve. They're usually installed when doing brake conversions such as rear disc or an aftermarket master cylinder when the factory combination valve is removed. It's supposed to be adjusted so that the rear brakes don't apply before the fronts. It's there to prevent the rear end of your car from passing you up on a hard stop!