Abstract: A parking brake control device has a differential area shuttle valve which has an outlet connected to a brake actuator and a larger area of a poppet directed to an inlet connected to a control valve for applying a parking brake pressure and a smaller area of the poppet facing a second inlet connected to a service brake pressure such that in the event of simultaneous application of parking brake and service brake pressures the shuttle valve will be moved to close off the inlet to the service brake pressure while introducing park brake pressure to the brake cylinder.

Abstract: In a brake system the actuator pressure is controlled using a closed loop feedback system where, a system pressure, indicated by a pressure transducer is compared with a predetermined reference value. During the individual control routines, pressures corresponding operation times of inlet and outlet valves are stored in a memory. In the event of a failure of the pressure transducer, a switch is made to an open loop control mode where, starting from a previously established reference pressure, the system pressure can be read from the memory and controlled by appropriate operation of the inlet or outlet valves, to achieve the desired level.