Brake Proportioning Valves
The proportioning valve is installed on vehicle's with front disc, rear drum brake systems. They provide balanced braking during sudden, hard braking by restricting fluid pressure to the rear brakes. This helps prevent rear wheel lock up as the vehicle's weight is shifted toward the front wheels.
The typical inline proportioning valve remains open until a panic brake situation occurs. During a sudden braking situation, high pressure from the master cylinder moves the larger piston, closing 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. Many proportioning valves are mounted directly in the master cylinder outlet to the rear brakes.
A height sensing proportioning valve is installed in the hydraulic line leading to the rear drums. They are found on many light trucks, located between the vehicle's chassis and the rear axle. When a driver brakes hard, the rear lifts up and forward. This lightens the load on the rear tires increasing the chance of lock up. It incorporates a lever attached to a spring that moves a valve to restrict fluid pressure as the vehicle's chassis raises up and 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.