Brake Proportioning Valves
The proportioning valves are found on vehicles with front disc rear drum brake systems. They keep the rear brakes from locking up during sudden, hard braking by restricting fluid pressure to the rear brakes. A faulty proportioning valve results in the rear brakes locking up with only light to moderate pressure applied to the brake pedal. They lock up as the vehicle's weight shifts 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 a valve and restricting fluid pressure. In late-model vehicles, these valves have been incorporated into the combination valve, metering valve, and failure warning light switch. Many proportioning valves are mounted directly in the master cylinder outlet to the rear brakes.
Height-sensing proportioning valves are connected to the hydraulic lines leading to the rear drums between the vehicle's chassis and the rear axle. When a panic braking occurs, the vehicle's rear end lifts up and forward, increasing the chance of rear wheel lock-up. It has a lever that moves a valve to restrict fluid pressure to the rear brakes 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. It utilizes ABS to prevent rear wheel lock-up.