See Figure 1
The rear wheel anti-lock system found on Toyota trucks is designed to keep the rear wheels from locking under hard braking regardless of the load being carried. This function aids in keeping the vehicle from skidding sideways under hard braking with loss of directional stability.
Since the anti-lock system acts only on the rear wheels, it may be possible to lock the front wheels, causing them to skid; no steering response is possible if this happens.
The electro-hydraulic system employs one speed sensor in the differential housing. The speed signal is sent to the Electronic Control Unit (ECU) which controls the operation of the control solenoid within the actuator. The solenoid controls brake line pressure within the single line running to there are axle; the line branches to each wheel at the axle housing.
A significant difference in this system (from 4 wheel ABS) is that power steering fluid is circulated through the brake system actuator. The ABS control solenoid acts on the power steering fluid, not the brake fluid circuit. The power steering fluid acts as a control pressure circuit, moving pistons and valves within the actuator which control the brake fluid circuit. The power steering fluid and the brake fluid are kept completely separate within the systems; each fluid is kept within its normal system reservoir.
Use of the power steering fluid allows a supply of constantly pressurized fluid (from the power steering pump) and eliminates the need for larger, bulkier actuators with electric pumps and accumulators.