Chevrolet Cobalt Service & Repair Manual: Description

Front & Rear Split System

When the brake pedal is depressed, both the primary (front brake) and the secondary (rear brake) master cylinder pistons are moved simultaneously to exert hydraulic fluid pressure on their respective systems.
If the rear brake system fails, initial brake pedal movement will cause the unrestricted secondary piston to bottom in the master cylinder bore. Primary piston movement will displace hydraulic fluid in the primary section of the master cylinder to actuate the front brake system.
If the front brake system fails, initial brake pedal movement will cause the unrestricted primary piston to bottom out against the secondary piston. Continued downward movement of the brake pedal moves the secondary piston to displace hydraulic fluid in the rear brake system to actuate the rear brakes.

Diagonally Split System

This system operates on the same principle as conventional front and rear split systems, using primary and secondary master cylinders which move simultaneously to exert hydraulic pressure on their respective systems. The hydraulic brake lines on this system, however, have been diagonally split front to rear (lefthand front to righthand rear and righthand front to lefthand rear) in place of separate lines to the front and rear wheels.
In the event of a system failure, the remaining non-failed system will do all the braking on one front wheel and one rear wheel, maintaining 50% of the total braking force.

Components

Warning Lamp

The warning lamp should illuminate when the ignition switch is in the start position and turn off when the switch returns to run. If the brake lamp remains on after the ignition returns to run, inspect fluid level in master cylinder reservoir and inspect parking brake. If the warning lamp does not turn on during cranking, inspect for faulty bulb or blown fuse.

Fluid Level Sensor

This sensor, mounted on the master cylinder, will activate the brake warning lamp if a low brake fluid level is detected. The lamp will turn off once the fluid level is corrected.

Brake Warning Lamp Switches

As pressure falls in one system, the other system's normal pressure forces the piston to the inoperative side, contacting the switch terminal, causing the warning lamp on the instrument panel to glow.
The switch is mounted directly in the master cylinder assembly. Whenever there is a specified differential pressure, the switch piston will activate the brake failure warning switch and cause the brake warning lamp to glow.

Combination Valve

The combination valve is a metering valve, failure warning switch, and a proportioning valve in one assembly and is used on disc brake applications. The metering valve delays front disc braking until the rear drum brake shoes contact the drum. The failure warning switch is actuated in event of front or rear brake system failure, in turn activating a dash warning lamp. The proportioning valve balances front to rear braking action during rapid deceleration.

Metering Valve

When the brakes are not applied, the metering valve permits the brake fluid to flow through the valve allowing the fluid to expand and contract with temperature changes.
When the brakes are initially applied, the metering valve stem moves to the left, preventing fluid to flow through the valve to the front disc brakes. This is accomplished by the smooth end of the metering valve stem contacting the metering valve seal lip at 4-30 psi. The metering valve spring holds the retainer against the seal until a predetermined pressure is produced at the valve inlet port which overcomes the spring pressure and permits hydraulic pressure to actuate the front disc brakes. The increased pressure into the valve is metered through the valve seal, to the front disc brakes, producing an increased force on the diaphragm. The diaphragm then pulls the pin, in turn pulling the retainer, and reduces the spring pressure on the metering valve seal. Eventually, the pressure reaches a point at which the spring is pulled away by the diaphragm pin and retainer, leaving the metering valve unrestricted, permitting full pressure to pass through the metering valve.

Failure Warning Switch

If the rear brake system fails, the front system pressure forces the switch piston to the right. The switch pin is then forced up into the switch, completing the electrical circuit and activates the dash warning lamp.
When repairs are made and pressure returns to the system, the piston moves to the left, resetting the switch. The detent on the piston requires approximately 100-450 psi to permit full reset of the piston. In event of front brake system failure, the piston moves to the left and the same sequence of events is followed as for rear system failure except the piston resets to the right.

Proportioning Or Pressure Control Valve

During rapid deceleration, a portion of vehicle weight is transferred to the front wheels. This resultant loss of weight at rear wheels must be compensated for to avoid early rear wheel skid. The proportioning or pressure control valve reduces rear brake system pressure, delaying rear wheel skid. When the proportioning or pressure control valve is incorporated in the combination valve assembly, pressure developed within the valve acts against the large end of the piston, overcoming the spring pressure, moving the piston left. The piston then contacts the stem seat and restricts line pressure through the valve.
During normal braking operation, the proportioning or pressure control valve is not functional. Brake fluid flows into the proportioning or pressure control valve between the piston center hole and the valve stem, through the stop plate and to the rear brakes. Spring pressure loads the piston during normal braking, causing it to rest against the stop plate.
On diagonally split brake systems, two proportioning or pressure control valves are used. One controls the lefthand rear brake, the other the righthand rear brake. The proportioning or pressure control valves are installed in the master cylinder rear brake outlet ports.

Brake Distribution Valve & Switch

This switch assembly is used on some diagonally split brake systems and Corvette four-wheel disc brake systems. It is connected to the outlet ports of the master cylinder and to the brake warning lamp and warns the driver if either the primary or secondary brake system has failed.
When hydraulic pressure is equal in both primary and secondary brake systems, the switch remains centered. If pressure fails in one of the systems, the piston moves toward the inoperative side. The shoulder of the piston contacts the switch terminal, providing a ground and lighting the warning lamp.
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    See also:

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