Antilock Brake System for Early Subarus:

Maximum Current Position (Active):

When the slip ratio approaches wheel lockup the ECU signals the HCU to energize the magnetic valves. The check valve to the master cylinder is Closed, and the check valve to the HCU reservoir is Open. This allows wheel cylinder pressure to flow to the HCU reservoir preventing wheel lockup. The pump motor is energized, and the pump operates at 2-3 strokes per second.

Each upward stroke of the pump transfers fluid to the accumulator for use during the No Current position. When system pressure exceeds the F valve spring pressure, the check valve seats, which prevents pressure flow to the master cylinder and prevents brake pedal kickback. Each downward stroke of the pump reduces fluid pressure to the HCU reservoir, which reduces pressure to the wheel cylinder.

No Current Position (Active):

When the ECU detects that wheel speed is resuming too quickly, a signal is sent to the HCU to de-energize the magnetic valve. The magnetic valve port opens allowing fluid pressure to flow to the wheel cylinder from the accumulator, which slows wheel speed. The cycle is repeated until wheel slip is controlled.