Hydraulic System: Description and Operation
Hydraulic Brake ActuationCOMPONENT LOCATION INTRODUCTION
The hydraulic brake system is a diagonally split dual line system, and consists of the brake pedal, brake master cylinder and hydraulic pipes and hoses. The system also features a non-active brake booster.
Brake pipes distribute pressure from the master cylinder to the 4 corners of the vehicle via the ABS module. Braided steel hoses are used to connect the brake pipes to the front and rear calipers.
BRAKE PEDAL
The brake pedal is mounted in a bracket attached to the rear side of the engine bulkhead. The bracket also contains the accelerator pedal. A clevis pin connects the brake pedal to the push rod of the brake booster.
The stoplamp switch is mounted in the brake pedal bracket and is operated by the brake pedal. The stoplamp switch is also used by the speed control system and the ABS.
BRAKE MASTER CYLINDER AND FLUID RESERVOIR
NOTE: Left-Hand Drive (LHD) shown, Right-Hand Drive (RHD) similar
The brake master cylinder is attached to the front of the brake booster, on the drivers side of the engine compartment. The brake master cylinder consists of a cylinder housing containing 2 pistons in tandem. The rear piston produces pressure for the primary braking circuit and the front piston produces pressure for the secondary circuit. A reservoir is mounted on top of the master cylinder to provide a supply of brake fluid for the primary and secondary circuits. The reservoir also provides a location for the brake fluid level switch.
BRAKE FLUID LEVEL SWITCH
The brake fluid level switch is located in the fluid reservoir and is hardwired to the ABS module. When the level of fluid in the reservoir reaches a predetermined low level, the switch contacts close and provide a signal feed back to the ABS module. The ABS module then broadcasts a message on the high speed Controller Area Network (CAN) bus to the instrument cluster. On receipt of the message, the brake fluid red warning indicator will illuminate and 'BRAKE FLUID LOW' will be displayed in the message center.
ANTI-LOCK BRAKING SYSTEM MODULE
The ABS module is located in the LH front wheel arch and incorporates the Hydraulic Control Unit (HCU). The HCU is a 4 channel unit that modulates the supply of hydraulic pressure to the brakes under control of the ABS module.
The primary and secondary outlets of the master cylinder are connected to the primary and secondary circuits within the HCU. The primary circuit in the HCU has separate outlet ports to the RH front and LH rear brakes. The secondary circuit in the HCU has separate outlet ports to the LH front and RH rear brakes.
CAUTION: The ABS module and the Hydraulic Control Unit (HCU) are a single unit and must not be separated.
CONTROL DIAGRAM
Hydraulic Control Unit
PRINCIPLES OF OPERATION
When 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 which, 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 in the master cylinder. Further movement of the pistons then pressurizes the fluid in the primary and secondary chambers and thus the brake circuits. The fluid in the chambers behind the pistons is unaffected by the movement of the pistons and can flow unrestricted through the inlet ports between the chambers and the reservoir.
Pressurized fluid enters the HCU, which is mounted on the front of the ABS module. The HCU modulates the supply of pressurized fluid to the brakes under control of the ABS module.