Abstract: The disclosed motor vehicle hydraulic brake system includes a master cylinder supplying fluid pressure to a front brake circuit and to a rear brake circuit. The rear brake circuit includes a proportioning valve which reduces the pressure supplied to the rear wheel brakes. The rear brake circuit also includes a control circuit. In the event of front and rear braking pressures exceeding predetermined pressure levels, the control circuit modifies the operation of the proportioning valve so that the proportional relationship of the proportioning valve closely approximates a 0.4g deceleration line until the theoretically ideal front to rear pressure relationship for the particular loading condition of the vehicle at the 0.4g deceleration rate is reached. The proportioning valve then closely approximates the theoretically ideal front to rear pressure relationship for the particular loading condition of the vehicle at all higher braking pressures and higher deceleration rates.

Abstract: A motor vehicle air brake system which includes a blend back proportioning valve which reduces brake sensitivity at low braking pressures and which allows full braking at high braking pressures. The proportioning valve includes a shuttle piston, a blend back sleeve, and a one-piece outlet diaphragm. The outlet diaphragm seals a reference chamber from outlet pressure and permits movement of the shuttle piston and blend back sleeve together as well as movement of the shuttle piston and blend back sleeve relative to one another. The proportioning valve also includes a one-piece inlet diaphragm which seals the reference chamber from inlet pressure and which provides a valving surface to open and close communication between the inlet and the outlet of the valve. In a second and third embodiment of the proportioning valve, springs are added to alter the proportioning characteristics of the valve.