Hydraulic System: Description and Operation

Hydraulic Brake Actuation

COMPONENT LOCATION

NOTE:
4.4L V8 vehicle shown, other vehicles similar

NOTE:
RHD (right-hand drive) installation shown, LHD (left-hand drive) installation similar









OVERVIEW
Hydraulic brake actuation consists of:
- The brake pedal.
- The brake master cylinder and reservoir.
- The hydraulic pipes and hoses.

BRAKE PEDAL









The brake pedal is mounted in a bracket attached to the rear side of the engine bulkhead. A clevis pin and clip connect the brake pedal to the push rod of the brake booster.
The stop lamp switch is mounted in the brake pedal bracket and operated by the brake pedal. For additional information, refer to Anti-Lock Control - Traction Control

BRAKE MASTER CYLINDER AND RESERVOIR









The master cylinder assembly produces hydraulic pressure to operate the brakes when the brake pedal is pressed. The assembly is attached to the front of the brake servo assembly, and comprises a cylinder containing two pistons in tandem. The rear piston produces pressure for the primary circuit and the front piston produces pressure for the secondary circuit.
When the brake pedal is pressed, the front push rod in the brake servo assembly pushes the primary piston along the cylinder bore. 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 cylinder bore. The initial movement of the pistons, away from the piston stops, closes the primary and secondary center valves.
Further movement of the pistons then pressurizes the fluid in the primary and secondary pressure chambers, and thus the brake circuits. The fluid in the chambers, behind the pistons, is unaffected by movement of the pistons and can flow unrestricted through the feed holes between the chambers and the reservoir. When the brake pedal is released, the primary and secondary springs push the pistons back down the bore of the cylinder. As the pistons contact the piston stops, the primary and secondary center valves open, which allows fluid to circulate between the reservoir, through the center valves, the chambers behind the pistons and the feed holes.
The brake fluid reservoir is located on top of the master cylinder assembly. The reservoir is internally divided to provide an independent supply of fluid to each brake circuit, and so prevent a single fluid leak from disabling both primary and secondary brake circuits. Should a failure occur in one circuit, the remaining circuit will still operate effectively, although brake pedal travel and vehicle braking distances will increase.
A fluid level switch is incorporated into the reservoir cap:
- With sufficient fluid in the reservoir the contacts of the fluid level switch are closed.
- When the fluid level drops below allowable limits, the switch contacts open.
Since the normal position is closed, this circuit is monitored for shorts to + and ground.
The ABS (anti-lock brake system) module monitors the input from the fluid level switch and issues a low fluid level signal to the LCM (lighting control module) on the CAN (controller area network) if the switch opens.
If the low fluid level signal is momentary, the LCM (lighting control module) signals the instrument cluster of the condition. The instrument cluster then informs the ABS (anti-lock brake system) module over the CAN (controller area network) bus. If the signal is received prior to the activation of the pre-charge pump, the pre-charge pump activation is delayed.
If the signal is present for more then 25 seconds, the LCM (lighting control module) issues two I bus messages for the instrument cluster:
- Display CHECK BRAKE FLUID in the message center.
- Notify the ABS (anti-lock brake system) module, via CAN (controller area network), that the fluid level has been low for more than 25 seconds.
The ABS (anti-lock brake system) module immediately switches DSC functions off and sends a CAN (controller area network) signal to the instrument cluster to continuously illuminate the DSC warning indicator.

BRAKE PIPES AND HOSES
The brake pipes and hoses connect the master cylinder to the wheel brakes via the hydraulic control unit. The pipes are arranged to provide a front and rear split braking system. The brakes on the front axle are operated by the primary system; the brakes on the rear axle are operated by the secondary system.