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.