The proportioning valve in a vehicle's brake system provides a way to control how much and when brake pressure is applied to each axle set of the brake system. Located between the master cylinder and the rest of the brake system, this component is the key to safe, reliable brake system operation in all conditions.
The benefit of having a proportioning valve in the system is that you can control front to rear brake bias by reducing pressure to the rear brakes when system pressure is high during rapid deceleration. This prevents premature rear brake lock-up during times of heavy braking. In vehicles with lightweight back ends, like pick up trucks, this is important.
There are three types of proportioning valve in use today: master cylinder mounted, combination valve mounted and load sensing. Master cylinder mounted valves are located on the master cylinder and screwed into the fluid ports between the master cylinder and the brake lines. Combination valve mounted valves are part of the combination valve along with the pressure differential switch and the metering valve. This component is usually mounted to the inner fender or frame, just below the master cylinder. Load sensing proportioning valves are located in the rear of the vehicle on the frame. A lever attaches the valve to the suspension.
The proportioning valve is a spring-loaded device. As fluid pressure builds, the plunger in the valve begins to unseat. When pressure reaches the calibrated range, the spring is compressed, and the plunger moves to block the fluid passage. This prevents premature wheel lock-up. In a load sensing valve, the same function applies, however, the arm attaching the valve to the suspension changes the calibration of the valve to compensate for load. As the load gets heavier and compresses the suspension, the amount of pressure supplied to the rear wheels is increased.
There are two types of combination valve in use today: two-way valves and three-way valves. The two-way valve can be identified by its roughcast housing. This type of valve may have a proportioning valve and pressure differential switch or a metering valve and a pressure differential switch. The three-way valve is a machined aluminum housing containing a metering valve, a proportioning valve and a pressure differential switch. The load-sensing valve can be identified by its location on the rear frame and suspension.
Late model vehicles equipped with ABS, traction control and electronic stability control do not use mechanical proportioning valves. Proportioning functions are handled through the ABS system; this gives greater control, in a wider variety of situations.
