How Does a Combination Valve Work?
A combination valve has several different functions according to the specific make, model and configuration of vehicle. A proportioning valve will control how much braking force is applied to the rear brakes of the vehicle and is designed to act as a safety measure whenever heavy pressure is applied in an emergency braking situation. The valve restricts how much pressure is applied to the rear brakes, without this feature the rear system would lock up and cause the car to go out of control.
If the car is equipped with a disc and drum combination, then a metering valve is also needed to regulate how much pressure is applied to the front wheels. As disc brakes (fitted to the front) are typically more efficient, then they will apply far more quickly in a heavy braking situation and this could cause the front of the vehicle to become unstable. Typically, a simple switch is also included within the valve housing, which can be activated should brake pressure be lost. When this happens, a light is illuminated on the dashboard instrument cluster to advise the driver that there is a problem with the braking system.