to minimize the power losses of the pump motor in a hydraulic brake system with brake slip control and traction slip control , a shut - off valve prevents flow in the suction line when the supply pressure of the pump is sufficient for traction slip control . this prevents the aspiration of further pressure fluid volume from the pressure fluid reservoir . the pump does not return any fluid volume into the pressure fluid reservoir by way of a pressure - relief valve so that the motor of the pump is relieved . preferably , the shut - off valve is designed so that the master cylinder pressure acts in the opening direction and the supply pressure of the pump acts in the closing direction of the shut - off valve , the valve being biassed in the opening direction by a compression spring .