A brake proportion valve is a valve that adjusts the rate of pressure rise to the back brakes. It keeps the brakes from locking up in hard braking conditions. It is installed in the rear brake line.
When the brakes are used, an adjustable proportioning valve can slow down the pressure rise that happens. Vehicles shift weight to the front wheels when under braking and the rear wheels lose weight and can lock up if the pressure is great too fast in the braking process. The proportioning valve applies full pressure to the rear brakes. If the rear wheels lock up easily, the brake proportion valve has gone bad.