Antilock Brake System for Early Subarus:
Moving The Plunger Piston:
When the system puts the right rear hydraulic circuit in pressure reduce mode, the solenoid valve in that circuit closes the inlet and opens the outlet. Wheel cylinder pressure is then reduced because brake fluid is allowed to bleed back to the reservoir in the HCU.
Through port 5, there is a hydraulic connection between the wheel cylinder circuit and the left half of the pressure chamber (zone B). With wheel cylinder pressure reduced, pressure acting on the left side of the plunger piston is also reduced. Master cylinder pressure acting on the right side of the plunger piston (zone A) now overcomes spring tension and begins to move the plunger piston to the left. As the plunger piston moves, the push rod causes piston 2 to move along with it.
As piston 2 moves to the left, it first closes the pressure port to isolate the left rear wheel cylinder (port 4) from master cylinder pressure (port 1). As piston 2 moves farther to the left, it exposes the left rear wheel cylinder (port 4) to the right side of the second chamber (zone D). As piston 2 continues to move to the left, the expanding volume in the second chamber decreases pressure in the left rear hydraulic circuit.
When the system once again allows pressure to increase in the right rear hydraulic circuit, pressure in zone B moves the plunger piston back to the right. In actual practice, this process of moving the piston happens very quickly and repeats many times per second as the system cycles.