The Rear Wheel Antilock Brake System (RWAL) is designed to prevent rear wheel lockup under heavy braking conditions. The main system components are: the RWAL valve, controller (CAB), rear wheel speed sensor mounted on the axle, and exciter ring on the differential.
When the brakes are applied, hydraulic fluid is routed from the master cylinder's secondary circuit through the combination valve, to the RWAL valve. From there the fluid is routed to the rear brake wheel cylinders. The CAB monitors rear wheel speed through the sensor on the axle. If a wheel is about to lock up, the CAB signals the RWAL valve which then modulated hydraulic pressure to the rear wheels to prevent lock up.
During light brake application, rear wheel deceleration is not sufficient to activate the antilock system. During a normal stop the brake fluid flows without restriction to the rear wheel cylinders to stop the vehicle. The antilock solenoids are inactive. The isolation valve is open and the dump valve is closed allowing normal fluid flow to the rear wheel cylinders.
If the CAB senses impending rear wheel lockup, it will energize the isolation solenoid. This prevents a further increase of driver induced brake pressure to the rear wheels. If this initial action is not enough to prevent rear wheel lock-up, the CAB will momentarily energize a dump solenoid. This opens the dump valve to vent a small amount of isolated rear brake pressure to an accumulator. The action of fluid moving to the accumulator reduces the isolated brake pressure at the wheel cylinders. The dump (pressure venting) cycle is limited to very short time periods (milliseconds). The CAB will pulse the dump valve until the rear wheel deceleration reaches the desired slip rate programmed into the CAB. The system will switch to normal braking once wheel-locking tendencies are no longer present.
Note that the RWAL system controls both rear wheel brakes simultaneously.