The Rear Wheel Anti-lock Brake System (RABS) is an electronically operated, hydraulically controlled rear wheel brake control system. Major components include the vacuum power brake booster, master cylinder, the speed sensor, and the control valve or Hydraulic Control Unit (HCU) and the control module.
The system is designed to retard rear wheel lockup during periods of high wheel slip when braking. Retarding the rear wheel lockup is accomplished by modulating fluid pressure to the rear wheel brake circuit. When the control module detects a variation in voltage across the speed sensor, the RABS is activated. The control module opens and closes various valves located inside the HCU/control unit. These valves, called dump and isolation valves, modulate the hydraulic pressure to the rear wheels by applying and venting the pressure to the brake fluid circuits.