Hydraulic Control Unit

The Hydraulic Control Unit (HCU) includes a front valve body and a separate rear valve body.

CONSTRUCTION
Within the front HCU are solenoids, valves, a pump, check valves, accumulators, and reset switches necessary to apply and release brake pressure as required to avoid wheel lockup, keep the wheels rolling, and maintain optimum deceleration. The rear HCU has similar internal components, but does not have a high pressure accumulator or pump/motor.

The Isolation Valves are normally open, allowing unrestricted flow from the master cylinder to the wheels. When the Anti-lock Brake Controller (CAB) determines antilock intervention is required, the valve closes to isolate the master cylinder hydraulic circuit. Fluid is trapped in the circuit and then prevented from reaching the wheels.

The Dump Valves are pulsed on and off by the CAB. They cycle only if the isolation valve is closed. When the dump is on, it allows fluid to the low pressure accumulator for temporary storage. This causes the pressure to the wheel to decrease. When the valve is off, fluid is allowed to the wheel.

The Low Pressure Accumulator (LPA) temporarily stores fluid purged from the hydraulic lines from the wheels. It is normally empty.

The High Pressure Accumulator (HPA) stores hydraulic fluid under pressure to be used during reapply. It fills when the CAB turns on the pump. The pump will run to fill the HPA only when a front ABS application is required.

The Pump Output Check Valve separates the master cylinder and pump to prevent unnecessary feeding of fluid to the HPA from the master cylinder during a high pressure brake apply. The ABS contains enough fluid to increase brake pressure to its level at the time the isolation valve is closed. It is a normally closed valve that opens when the pump pressure exceeds the spring force and fluid flows out to the brakes during a re-apply. To get additional pressure, more fluid must come from the master cylinder. The driver may notice a slight pedal drop.

The Hydraulic Check Valve also prevents pedal feedback and does not allow the pump pressure to exceed the pedal pressure. It is a normally open valve that closes when the pump pressure shuttles the switch inside due to a pressure differential during the ABS stop.

The Brake Return Check Valve allows the HCU to drain faster after the ABS stop, when the brake is released.

The Internal Bleed Screw is open when the HCU is shipped to the assembly plant. After evacuation and fill, the bleed screw is closed. By opening the bleed screw, the passage ways in the bottom portion of the valve are open, and bleeding the valve becomes much easier during a base brake repair.

The Reset Switch monitors the hydraulic circuits for an imbalance in system pressure. When the valve body has a difference in pressures, it will be forced to slide in its bore, closing the electrical contacts and alerting the system of a pressure imbalance. In the non-antilock mode, the reset switch alerts the module to any system pressure imbalance, as in the case of a hydraulic failure in the base brake system.

The Pump is driven by the motor to provide fluid movement under high pressure for the HPA circuit. The pump will never create more pressure than what is demanded by the driver through his brake pedal input.