A motor vehicle fluid power circuit is provided with a constant displacement pump, a control valve, an open center steering valve, a hydraulic ratio type brake booster, and a ratio override valve. The control valve includes a shuttle piston which controls fluid flow between the pump and the brake booster, between the pump and the steering valve, and between the brake booster working chamber and the downstream side of the steering valve. The brake booster includes a trapped volume chamber in which a larger area input rod moves to provide greater movement of an output rod and power piston assembly.