A proportioning valve compensates for timing differences in the deployment of disc brakes and drum brakes in cars that have both. Without proportioning, the disc brakes would engage before the drum brakes. The proportioning valve provides variable pressure for even braking.
A proportioning valve is a requirement in cars that have disc brakes on the front wheels and drum brakes on the back wheels. Disc brake pads normally remain in contact with the discs but drum brake pads do not remain in contact with the drums, which can cause uneven braking or locking up of the rear wheels. By controlling the pressure to each set of brakes, a proportioning valve prevents unsafe braking.