Abstract: A motor vehicle fluid power circuit is disclosed which includes a primary pump, a closed center brake booster, and an open center steering valve. These components are hydraulically connected in series with the fluid flowing from the pump to the closed center brake booster and then to the open center steering valve. The brake booster includes valve means throttling fluid flow from the pump to the steering valve and maintaining the outlet pressure of the pump at at least a predetermined pressure differential above the working pressure of the brake booster under normal operating conditions in order to conserve output load requirements of the pump while assuring adequate pump pressure for brake booster actuation. In a second embodiment, a back-up pump is also provided in the circuit, and a pilot valve hydraulically disconnects the primary pump and hydraulically connects the back-up pump to the brake booster in response to a fluid pressure failure in the primary pump.

Abstract: A motor vehicle fluid power circuit is disclosed in which a pump driven by the engine of the vehicle assists the operator in braking the vehicle and in steering the vehicle. The circuit includes a pump, a brake booster, and a steering valve. A valve spool throttles fluid flow from the pump to the steering valve to maintain the outlet pressure of the pump at least a predetermined pressure differential above the working pressure of the brake booster. The brake booster includes a shut-off means which hydraulically isolates the pressure side of the pump from the brake booster when the brake booster reaches a predetermined shut-off pressure which is less than the relief pressure of the pump relief valve so that operation of the brake booster cannot actuate the relief valve of the pump.