Brakes and Traction Control: Description and Operation

Foundation Brakes

General
The brake system is a dual circuit type with Electronic Brake Force Distribution (EBD) through DSC/ABS control. The system comprises:
^ Vacuum booster
^ Tandem master cylinder with two central valves for DSC operation
^ Brake fluid reservoir with reed contact fluid level switch
^ Vented front discs/Solid rear discs
^ Single piston calipers
^ ABS hydraulic unit with integrated control module
^ Pre-charge pump for DSC operation.





Front Brakes
The front brakes each comprise a hub mounted, single piston caliper assembly and a vented disc. The inboard side of the disc is protected by a mud-shield. When hydraulic pressure is supplied to the caliper, the piston extends and forces the inner pad against the disc. The caliper body reacts and slides on the guide pins to bring the outer pad into contact with the disc.





Rear Brakes
The rear brakes each comprise a hub mounted, single piston caliper assembly end and a solid disc. The operation is the same as for the front brakes.

Both the left front inboard and right rear inboard brake pad wear is monitored by a sensor attached to the pad by a fly lead. If the brake pad warning has been triggered the sensor and brake pads must be replaced.

Master Cylinder
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.

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.

If the fluid level in the reservoir is too low, the brake fluid level switch in the reservoir filler cap breaks a contact to the DSC ECU, which sends a CAN message to illuminate the brake warning lamp in the instrument pack and displays the message "CHECK BRAKE FLUID" in the instrument pack.





Brake Master Cylinder, Servo and Fluid Reservoir

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Part 2 Of 2:

Hydraulic 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 unrestricted between the two hydraulic circuits and the reservoir, through the center valves, the chambers behind the pistons and the feed holes.

Brake Servo
The brake servo assembly provides power assistance to reduce the pedal load when braking. The assembly is attached to the front of the pedal box and comprises a circular housing containing a diaphragm, push rods, valve and filter. A push rod at the rear of the housing is connected to the brake pedal.

The vacuum line is connected in a portion the front face of the housing. With the brake pedal released end the engine running, vacuum pressure is present on both sides of the diaphragm. When the brake pedal is pressed, the rear push rod moves forward and opens the valve to allow atmospheric pressure through the filter into the pedal side of the diaphragm The pressure differential acting on the diaphragm increases the pressure being applied by the brake pedal, which is transmitted to the master cylinder via the forward push rod. If the servo fails, the hydraulic system will still function but will require greater pedal effort due to the lack of vacuum assistance.