Abstract: A parking brake system (10) for a vehicle. The control system (10) includes solenoid valve (30) which controls communicating of pressurized fluid from an inlet port (32) to the actuator assembly (12) of a drum brake and from the actuator assembly (12) to a reservoir (41). Solenoid valve (30) has a coil (50) consisting of a first circuit (52) and a second circuit (54). In response to an operator activating a parking brake switch (80) and after signals corresponding to the operational condition of the vehicle are evaluated by an electronic control unit (100) current may directly flow in the first circuit (52) from a battery (200) to the solenoid valve (30) and may flow in a second circuit (54) after the closure of an ignition switch (201).

Abstract: A back-up power assisted braking system substitutes an intermittently actuated secondary source of pressurized hydraulic fluid such as a traction control/anti-lock pump and motor (41) for the normal braking power assist upon the detection of a failure in the primary power assisted braking system. A failure may be detected in responsive to sensing simultaneous brake pedal (23) displacement and a lack of fluid flow for providing an abnormality signal. Also, a failure may be detected by sensing a predetermined difference between the commanded braking force (45) and the actual braking forces (38, 49). There is a pressure transducer (45) which provides an indication of brake pedal commanded braking force and a hydraulic circuit (43, 53 or 55, 51) interconnecting the secondary source outlet, the individual wheel brake actuators (14) and secondary source inlet.