System Description
Principles of OperationWARNING:
^ Brakes contain asbestos fibers. Avoid breathing dust. Breathing asbestos dust may cause asbestosis and cancer. Breathing asbestos dust is hazardous to your health.
^ Dust and dirt present on wheel brake and clutch assemblies may contain asbestos fibers that are hazardous to health when made airborne by cleaning with compressed air or by dry brushing.
^ Wheel brake assemblies and clutch facings should be cleaned using a vacuum cleaner recommended for use with asbestos fibers such as Rotunda Asbestos Brake Drum System With Vacuum 091-00025. Dust and dirt from vacuum should be disposed of in a manner that prevents dust exposure such as sealed bags. The bag must be labeled as per Occupational Safety and Health Act (OSHA) instructions and the trash hauler notified as to the bag's contents.
^ If a vacuum suitable for asbestos is not available, cleaning should be done wet. If dust generation is still possible, technicians should wear government-approved toxic dust purifying respirators.
^ Grinding or sanding on brake linings, pads, rotors, drums or clutch facings should use properly exhaust-ventilated equipment.
^ OSHA requires areas where asbestos dust generation is possible to be isolated and posted with warning signs. Only technicians concerned with performing brake or clutch service should be present in the area.
^ Brake fluid contains polyglycol ethers and polyglycols. Avoid contact with eyes. Wash hands thoroughly after handling. If brake fluid contacts eyes, flush eyes with running water for 15 minutes. Get medical attention if irritation persists. If taken internally, drink water and induce vomiting. Get medical attention immediately.
CAUTION: Do not spill brake fluid onto painted surfaces. If spilled, wipe up immediately.
Rear Drum
The rear drum brakes use an internal hydraulic rear wheel cylinder with expanding brake shoes and linings that are applied against a rotating brake drum. An incremental brake adjuster screw is actuated whenever sufficient wear occurs Brake adjustment takes place in forward or reverse braking, but not with parking brake application.
Front Disc Brake Caliper
The front disc caliper consists of a pin slider-type front disc brake caliper housing, inner and outer front brake shoes and linings, caliper piston, dust seal and piston seal. The two caliper bolts attach the front disc brake caliper to the front wheel knuckle.
Front Disc Brake Rotor
The front disc brake rotor is attached to the hub. The braking surfaces of the front disc brake rotor are separated by nondirectional cooling fins.
Power Brake Booster
The power brake booster uses engine vacuum for operation. The applied force created when the brake pedal is depressed is mechanically increased with the assistance of the engine vacuum. Power brakes reduce the amount of pedal pressure necessary to stop the vehicle, as well as reducing brake pedal travel (the distance from the release position to full brake application).
The power brake booster is divided into two chambers by a rubber diaphragm. The A-chamber is on the master cylinder side (front) of the power brake booster, and the B-chamber is on the brake pedal side (rear) of the power brake booster. A vacuum exists in both chambers when the brake pedal is in the released position. When the brake pedal is depressed, the air control valve is forced against the booster piston seat. This action closes the vacuum valve between the two chambers, and the vacuum to the B-chamber (rear) is cut off. As the brake pedal is further depressed, the air control valve allows atmospheric pressure to enter the B-chamber. This creates a pressure difference between the two chambers. This pressure difference causes the diaphragm, booster piston and air control valve to move forward into the A-chamber, pushing the piston rod into the brake master cylinder and applying pressure to the braking system.
When the brake pedal is released, the booster piston return spring forces the diaphragm, booster piston and air control valve rearward, closing the air intake valve and opening the vacuum valve between the two chambers. A common vacuum now exists between the two chambers again.
Brake Master Cylinder
The brake master cylinder creates hydraulic pressure that is transferred through the brake tubes and hoses. This forces the caliper pistons and the rear wheel cylinder pistons to move against the front brake shoes and linings, and the rear brake shoes and linings to contact the metal braking surfaces.
The brake master cylinder contains dual hydraulic cylinders with a primary and a secondary hydraulic piston.
The hydraulic brake system is a diagonally split system. The left front and right rear are on one brake circuit, while the right front and left rear are on another brake circuit.
Brake Load Leveling Proportioning Valve
When the vehicle is unloaded, the brake load sensor proportioning valve closes and partially reduces hydraulic pressure to the rear wheel cylinders. As the vehicle is loaded, the weight presses down the back end of the vehicle on the rear springs. This activates the brake load leveling proportioning valve, which opens and supplies more hydraulic pressure to the rear wheel cylinders. When the vehicle is fully loaded, the brake load leveling proportioning valve is fully open and provides full hydraulic pressure to the rear wheel cylinders.
Check Valve
A power brake booster check valve closes when the engine is shut off. The closed power brake booster check valve traps engine vacuum in the power brake booster. This reverse vacuum allows several assisted brake applications without the engine running.
If any of the vacuum-related parts fail, causing the power brake booster to fail, the brakes will still operate. The force of the brake pedal being depressed will still be transferred through the control valve, booster piston, piston rod and on to the brake master cylinder. The brake system will still operate, but the brake pedal will require more effort to stop the vehicle.