The proportioning valve can also be known as a bias valve.
Proportioning valves reduce the pressure to the rear brakes. It does not matter what type of brakes a car has, the rear brakes require less force than the front brakes.
If equal brake pedal force were applied to all four wheels during a stop the rear wheels would lock up before the front wheels. The proportioning valve only lets a certain portion of the pressure through to the rear wheels so that the front wheels apply more braking force or pressure.
Although most proportioning valves look as if they are totally sealed units CBC can overhaul most of these units. They have quite different seals and we have had specific seals made to complete this requirement.