A proportional pressure difference valve is disclosed between a master cylinder and normally open valves for supplying hydraulic pressure discharged from the master cylinder into wheel brake cylinders, which are connected to normally closed valves for reducing pressure. The normally open valve connected to one of the wheel brake cylinders in one hydraulic circuit is placed in its closed position, and the normally open valve connected to the other one of the wheel brake cylinders is placed in its open position and then the pressure difference valve is controlled on the basis of a monitored vehicle state variable to regulate a pressure difference between the pressure at the side of the master cylinder and the pressure at the side of the normally open valves.