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Hydraulic System: Description and Operation





Hydraulic Brake Actuation

COMPONENT 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 brake pipes distribute pressure from the master cylinder to the four corners of the vehicle via the ABS (anti-lock brake system) 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 operated by the brake pedal. There are two switches in the stoplamp switch, which close when the brake pedal is pressed to output 'brake applied' signals. When closed, one switch connects a ground to the ECM (engine control module). The second switch connects battery power from the CJB (central junction box) to the:
- AJB (auxiliary junction box)
- CJB (central junction box)
- ECM (engine control module)
- EPB (electronic parking brake) module.

BRAKE MASTER CYLINDER AND FLUID RESERVOIR

NOTE:
LHD (left-hand drive) version shown, RHD (right-hand drive) version 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 two pistons in tandem. The rear piston produces pressure for the primary braking circuit and the front piston produces pressure for the secondary circuit.
The fluid reservoir is mounted on top of the master cylinder to provide a supply of brake fluid for the primary and secondary circuits.

Brake Fluid Level Switch
The brake fluid level switch is located in the fluid reservoir and is hardwired to the instrument cluster. When the level of fluid in the reservoir reaches a predetermined low level, the switch contacts close and provide a return signal to the instrument cluster, which illuminates the red brake warning indicator, and displays the red warning triangle and BRAKE FLUID LOW in the instrument cluster message center.

HYDRAULIC CONTROL UNIT









The HCU (hydraulic control unit) is attached to the ABS (anti-lock brake system) module in the LH (left-hand) front wheel arch. The HCU (hydraulic control unit) is a four channel unit that modulates the supply of hydraulic pressure to the brakes under control of the ABS (anti-lock brake system) module.

CAUTION: The ABS (anti-lock brake system) module and the HCU (hydraulic control unit) are a single unit and must not be separated. There are no serviceable parts within the brakes modulator.
The primary and secondary outlets of the master cylinder are connected to the primary and secondary circuits within the HCU (hydraulic control unit). The primary circuit in the HCU (hydraulic control unit) has separate outlet ports to the RH (right-hand) front and LH (left-hand) rear brakes. The secondary circuit in the HCU (hydraulic control unit) has separate outlet ports to the LH (left-hand) front and RH (right-hand) rear brakes.
Each of the HCU (hydraulic control unit) circuits contains the following components to control the supply of hydraulic pressure to the brakes:
- A normally open, solenoid-operated pilot valve, to enable active braking.
- A normally closed, solenoid-operated priming valve, to connect the brake fluid reservoir to the dual circuit return pump during active braking.
- A dual circuit return pump, driven by an electric motor, to generate hydraulic pressure for active braking and return brake fluid to the reservoir.
- Normally open, solenoid-operated inlet valves and normally closed, solenoid-operated outlet valves, to modulate the hydraulic pressure in the individual brakes.
- An accumulator and a relief valve, to allow the fast release of pressure from the brakes.
- Filters, to protect the internal components from contamination.

Hydraulic Control Unit Schematic









PRINCIPLES OF OPERATION

Brake Master Cylinder
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.

Hydraulic Control Unit
The HCU (hydraulic control unit) has three operating modes:
- Normal braking/EBD (electronic brake force distribution)
- ABS (anti-lock brake system) braking
- Active braking.

Normal Braking/EBD
Initially, all of the solenoid-operated valves are de-energized. Operating the brake pedal produces a corresponding increase or decrease of pressure in the brakes, through the open pilot valves and inlet valves. If the ABS (anti-lock brake system) module determines that EBD (electronic brake force distribution) is necessary, it energizes the inlet valves for both rear brakes, to isolate the brakes from any further increase in hydraulic pressure.

ABS (anti-lock brake system) Braking
If the ABS (anti-lock brake system) module determines that ABS (anti-lock brake system) braking is necessary, it energizes the inlet and outlet valves of the related brake and starts the return pump. The inlet valve closes to isolate the brake from pressurized fluid; the outlet valve opens to release pressure from the brake into the accumulator, and the inlet side of the return pump. The reduced hydraulic pressure allows the wheel to accelerate. The ABS (anti-lock brake system) module then operates the inlet and outlet valves to modulate the pressure in the brake to apply the maximum braking effort without locking the wheel. Control of the valves for each wheel takes place individually.

Active Braking Mode
The active braking mode is used to generate and control hydraulic pressure to the brakes for functions other than normal braking/EBD (electronic brake force distribution) and ABS (anti-lock brake system) braking, i.e. DSC (dynamic stability control), CBC (corner brake control), ETC (electronic traction control), EBA (emergency brake assist), understeer control and electronic brake prefill (on vehicles with ASC (adaptive speed control)). For additional information, refer to Anti-Lock Control - Stability Assist - 5.0L Description and Operation
For active braking, the ABS (anti-lock brake system) module energizes the pilot valves and priming valves, starts the return pump and energizes the inlet valves and outlet valves when required. Brake fluid, drawn from the reservoir through the master cylinder and priming valve, is pressurized by the return pump and supplied to the inlet valves. The ABS (anti-lock brake system) module then operates the inlet valves and outlet valves, as required, to modulate the pressure in the individual brakes. Some noise may be generated during active braking.

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