Description

Hydraulic Brake Actuation

PRINCIPLES OF OPERATION
As the brake pedal is pressed, the front push rod in the brake booster pushes the master cylinder primary piston along the bore of the housing. This produces pressure in the primary pressure chamber and in conjunction with the primary spring, overcomes the secondary spring and simultaneously moves the secondary piston along the bore.
The initial movement of the pistons away from the piston stops closes the primary and secondary center valves located inside the master cylinder. Further movement of the pistons then pressurizes the fluid in the primary and secondary chambers and the brake circuits. The fluid in the chambers behind the pistons is unaffected by the movement of the pistons and will flow unrestricted through the inlet ports, between the chambers and the reservoir.
Pressurized fluid enters the HCU (hydraulic control unit). The HCU (hydraulic control unit) then modulates the supply of pressurized fluid to the brakes under control of the ABS (anti-lock brake system) module. For additional information, refer to Anti-Lock Control - Traction Control Locations

BRAKE PEDAL









The brake pedal is mounted in a bracket attached to the rear side of the engine bulkhead. The brake pedal is connected to the brake booster push rod with a clevis pin.
The stoplamp switch and the brake diagnostic switch are mounted on the brake pedal bracket and operated by a plate attached to the brake pedal. Both switches are single pole switches hardwired to the CJB (central junction box), to give independent signals of brake pedal operation.

BRAKE MASTER CYLINDER AND FLUID RESERVOIR









The brake master cylinder is attached to the front of the brake booster, on the driver's side of the engine compartment. The brake master cylinder is a tandem design that supplies pressure to two independent (primary and secondary) hydraulic circuits. Each circuit is connected between the calipers of diagonally opposing front and rear wheels to provide a fail safe brake system.
A reservoir is mounted on top of the master cylinder and retains sufficient volume of hydraulic fluid to allow for normal system use, and to compensate for the replenishment of the system as the brake linings wear. The reservoir is internally divided to provide an independent supply of fluid to each brake circuit, and prevents a single fluid leak from disabling both primary and secondary brake circuits. If a failure occurs in one brake hydraulic circuit the remaining circuit will still operate effectively, although brake pedal travel and vehicle braking distances will increase.
On vehicles with manual transmission, the brake fluid reservoir also supplies hydraulic fluid for operation of the clutch assembly.

Brake Fluid Level Switch









The reservoir incorporates a brake fluid level switch that is hardwired to the CJB (central junction box). When the reservoir fluid falls to a predetermined low level, the switch contacts close and provide a signal feed back to the CJB (central junction box). The CJB (central junction box) then sends a message on the high speed CAN (controller area network) bus to the instrument cluster, which illuminates the brake fluid warning indicator lamp. For additional information, refer to Instrument Cluster Locations

HYDRAULIC CONTROL UNIT









The HCU (hydraulic control unit) is located on the rear LH (left-hand) side of the engine compartment and is an integral component with the ABS (anti-lock brake system) module. The HCU (hydraulic control unit) is a four channel unit that modulates the supply of hydraulic pressure to the brakes under the control of the ABS (anti-lock brake system) module.
The primary and secondary outlet ports of the master cylinder are connected via 8 mm (0.315 in) diameter brake pipes to the primary and secondary circuits of the HCU (hydraulic control unit). The primary circuit in the HCU (hydraulic control unit) provides two separate outlet ports to the RH (right-hand) front and LH (left-hand) rear brakes. The secondary circuit in the HCU (hydraulic control unit) provides two separate outlet ports to the LH (left-hand) front and RH (right-hand) rear brakes.
The HCU (hydraulic control unit) incorporates three brake operating modes as follows:
- Normal braking / EBD (electronic brake force distribution).
- ABS (anti-lock brake system) braking.
- Active braking.
As the HCU (hydraulic control unit) and ABS (anti-lock brake system) module is a fully integrated unit, details of the HCU (hydraulic control unit) and ABS (anti-lock brake system) functionality, including the three braking modes, are contained in the Anti-Lock Control - Traction Control section of the manual. For additional information, refer to Anti-Lock Control - Traction Control Locations

Servicing Information
The ABS (anti-lock brake system) module and HCU (hydraulic control unit) form a single component and must not be separated. The ABS (anti-lock brake system) module and HCU (hydraulic control unit) assembly is supplied in a pre-filled state. After installation, the hydraulic brake system only requires a conventional bleed of the system; there is no requirement to pressure bleed the system.