proportioning Valve

[TommyG]

Proportioning valve....What does it do, and is it necessary..?

[taylor351]

It proportions the pressure of the brake lines to the rear brakes.
From article in Chevy High Performance
Hydraulic brakes work on a very simple principal: Load is applied to the brake pedal, which creates pressure using hydraulic fluid that is directed through high-pressure lines to either a drum wheel cylinder or a disc brake caliper. For disc brakes, this pressure (in pounds per square inch—psi) is applied to one or more pistons that squeeze brake pads against a rotating disc. For drum brakes, the pistons in the wheel cylinders push outward to apply shoes against a rotating drum. Drum brakes benefit from something called self-energizing action where the leading shoe uses the rotating drum to help push the larger trailing shoe into the drum with additional force, requiring less hydraulic pressure to apply the brakes.

After disc brakes began appearing on production cars, the factory came up with a device called a combination valve. This valve is placed downstream of the master cylinder and performed both as a proportioning valve to reduce the pressure to the rear brakes while also acting as a failure warning device to trigger a light should a loss in hydraulic pressure occur. Factory combination valves are carefully engineered to reduce pressure to the rear brakes to balance the braking effort for maximum braking efficiency. Unfortunately, this device is not adjustable, which means any change to brake or suspension components may create a premature rear-brake lockup problem. Ideally, the front and rear brakes lock up at the same time.

Whether the car is equipped with front disc and rear drums, or has four-wheel discs, hydraulic pressure should be reduced to the rear brakes for several reasons. The single most important reason is because even under moderate braking, natural weight transfer reduces weight on the rear tires and moves the weight to the front. This unloads the rear tires, which makes it much easier to lock up the rear brakes in hard braking. Once the rear brakes are locked, the tires skid, which immediately causes loss of control.

This allows your rear brakes not to lock up before you front brakes

Keith