Hydraulic Control Unit (HCU)
The Hydraulic Control Unit (HCU) consists of a valve body assembly, pump and a motor assembly. During normal braking, fluid from the brake master cylinder enters the HCU through two inlet ports located on the bottom of the HCU. The fluid then passes through four normally open Isolation (ISO) valves, one to each of the four wheels. If the anti-lock brake control module senses that a wheel is about to lock, the anti-lock brake control module pulses the appropriate isolation valve, which in turn closes that valve. This prevents any more fluid from entering the effected brake. The anti-lock brake control module then looks at that wheel again. If it is still decelerating, the anti-lock brake control module pulses the normally closed dump valve, which decreases the pressure trapped in the line.