The proportioning valve’s job is to control the brake line pressure in the front and rear brakes. Simply put, if the brake balance between the front and rear brakes are not correct, you’re going to lock up the rear brakes and cause the rear wheels to skid.
Usually you want the front brakes to receive more braking power than the rears. The reason for this is because of the car nose-diving when you hit the brakes. Braking tends to force down the front wheels, and lifts the rear end of the car up. Typically rear wheel drive cars have 60-70% of the brakes geared toward the front. In front wheel driven cars, this can be from 80-90% as the front wheels are also the drive wheels.
Here is a photo of the proportioning valve on a 1988 Camaro.
Inside the proportioning valve is a spring loaded piston that determines how much pressure goes fore and aft. You can see the nut at the very front in this photo. It’s important to buy the same proportioning valve that came with the same model/optioned vehicle if your original valve is defective.
The reason for this is that the factory has it precision calibrated to that specific optioned brake in your vehicle and it’s pretty darn close!
For example, there is a difference between proportion valves in the drum brake F-body cars and the four wheel disk cars. (Even between the non-performance delco morraine brakes and the “performance” 1LE brakes, there is a slight difference.)
When the hydraulic pressure is applied to the wheel cylinder inside a drum brake ,the shoes are pushed out towards the drum. When the shoes make contact, the rotation of the drum tries to drag the the shoes along, however since the shoes are anchored in place, the drum only pulls the shoes tighter. This is why drum brakes are called “self-energizing” and require little additional pedal effort once applied.
Disc brakes are not self-energizing. It requires increased pedal effort to squeeze the pads against the rotor. Thus, the proportioning valve is different between the two cars.
Here is an interesting article from Stop-Tech regarding brake bias and the proportioning valve:
Here are some factors that affect brake bias from Stop-Tech’s site:
“Factors that will increase front bias
- Increased front rotor diameter
- Increased front brake pad coefficient of friction
- Increased front caliper piston diameter(s)
- Decreased rear rotor diameter
- Decreased rear brake pad coefficient of friction
- Decreased rear caliper piston diameter(s)
- Lower center of gravity (i.e. lowered vehicle)
- More weight on rear axle (i.e. loaded)
- Less weight on front axle
- Less sticky tires (lower deceleration limit)
Factors that will increase rear bias
- Increased rear rotor diameter
- Increased rear brake pad coefficient of friction
- Increased rear caliper piston diameter(s)
- Decreased front rotor diameter
- Decreased front brake pad coefficient of friction
- Decreased front caliper piston diameter(s)
- Higher center of gravity (i.e. raised vehicle)
- Less weight on rear axle (i.e. unloaded)
- More weight on front axle
- More sticky tires (higher deceleration limit)”
Generally, it’s best to leave your stock proportioning valve alone if you don’t have the testing equipment to get it right! Stop-Tech seems to agree.
On the drum brake F-bodies, you may also have a metering Valve. This just holds off or delays the hydraulic pressure to the front disc brakes until the rear drums start to work. I’m not sure if they actually combined that with the proportioning valve, when they do it’s called a combination valve.
Although there are magazines articles that say you can fool around with the spring inside the stock proportioning valve to get the back brakes to lock up sooner-the theory being that the rear brakes are not working hard enough, it goes against the fact that your front end nose-dives during braking, thus the front actually needs more brakes! GM brakes have a specific proportioning valve for every vehicle they manufacture and GM usually had it very close to what the car needs!