The proportioning valve is installed on vehicles with front disc rear drum braking systems. It is installed in the hydraulic line leading to the rear drum brakes and helps prevent rear wheel lock up. Many proportioning valves are mounted directly in the master cylinder outlet to the rear brakes. They provide balanced braking during panic braking situations or when the vehicle is loaded by restricting fluid pressure to the rear brakes. This helps prevent rear wheel lock up as the vehicles weight is shifted toward the front wheels.
The typical inline proportioning valve remains open until a panic brake situation occurs. When sudden high pressure from the master cylinder moves the larger piston it closes the valve restricting fluid pressure.
In late model vehicles these valves have been incorporated into the combination valve along with the metering valve and failure warning light switch. As mentioned earlier they are often threaded into the master cylinder outlet ports to the rear lines.
Another common proportioning valve is the load sensing proportioning valve. The load sensing proportioning valve is usually found on trucks and is located between the vehicles chassis and the rear axle. When a driver brakes hard in a pickup truck the rear end tends to lift up and forward. This lightens the load on the rear tires increasing the chance of lock up. This valve incorporates a lever attached to a spring that moves the valve to restrict fluid pressure as the vehicles chassis raises away from the axle.
An electronic brake proportioning EBP has replaced the conventional proportioning valve in many late model vehicles. This system monitors the speed difference between the front and rear wheels and utilizes the ABS system to prevent rear wheel lock-up.