Master Cylinder Power Unit Assembly

The Anti-Lock Brake System (ABS) master cylinder power unit assembly contains the necessary valves and pistons to develop hydraulic pressure in the wheel circuits during brake application. Power brake assist is provided through the use of pressurized brake fluid supplied by the hydraulic accumulator.

The ABS master cylinder applies hydraulic pressure to the LF and RF wheel cylinders. During non-antilock braking, the master cylinder operates as follows:
^ The reservoir provides fluid under atmospheric pressure to the master cylinder piston(s).
^ The booster piston force provides the master cylinder piston force to apply the LF and RF brakes. The ABS main valve allows booster hydraulic pressure to assist in applying the front brakes during antilock braking.

As part of its antilock control, the Electronic Brake Control Module (EBCM) completes the main valve solenoid circuit. This causes the master cylinder to operate as follows:
^ The main valve blocks the reservoir-to-master cylinder passage and opens the booster-to-master cylinder passage.
^ Booster hydraulic pressure directly applies the LF and RF channels, keeping the channels full of pressurized fluid during the cycling of the inlet and outlet valves.
A driver feels the effect of main valve operation during antilock braking when the brake pedal rises slightly against foot pressure.