How to Service a Vehicle's Hydraulic System
By Steven Lye
Vehicle Hydraulic System service is relatively uncomplicated, but it is vital to the vehicle's safe operation.
Brake Fluid Inspection
The master cylinder is usually located under the hood and near the fire wall on the driver's side. Remove the cover and check the gasket, or diaphragm. Inspect the cover for damage or plugged vent holes. Clean the vent holes, if necessary.
Check the brake fluid level in the master cylinder. A cast-iron reservoir is usually filled to within 1/4 inch of the top. A plastic reservoir may have fluid level mars. Do not overfill a reservoir. If fluid must be added, a leak probably has developed or the shoes and/or pads are worn. Check the system carefully to locate the leak.
To check for contaminated fluid, place a small amount of brake fluid in a clear glass jar. If the fluid is dirty or separates into layers, it is contaminated. Contaminated fluid must be replaced.
Contaminated brake fluid can damage rubber parts and cause leaks. When replacing contaminated brake fluid, it is necessary to flush and refill the brake system with new fluid. Always use fluid with a DOT rating of 3 or higher. Follow manufacturer's recommendations.
Check the master cylinder for dampness and leaks around the body fittings, especially at the rear. A leak where it is mounted to the fire wall or power brake unit indicate a defective rear piston seal. The master cylinder must be rebuilt or replaced.
Brake Line Inspection Check all tubing, hoses, and connections from under the hood to the wheels for leaks and damage. Wheels and tires should also be inspected for signs of brake fluid leaks. Check all hoses for flexibility, bulges, and cracks. Check parking brake linkage, cable and connections for damage and wear. Replace parts where necessary.
Brake Pedal Inspection Depress and release the brake pedal several times (engine running for power brakes). Check for friction and noise. Pedal movement should be smooth, with no squeaks from the pedal or brakes. The pedal should return quickly when it is released.
When operating the engine, be sure the transmission lever is in neutral or park. Be sure the area is properly ventilated for the exhaust to escape.
Apply heavy foot pressure to the brake pedal (engine running for power brakes). Check for a spongy pedal and pedal reserve. Spongy pedal action is springy. Pedal action should feel trim. Pedal reserve is the distance between the brake pedal and the floor after the pedal has been depressed fully. The pedal should not go lower than 1 or 2 inches above the floor.
With the engine off, hold light foot pressure on the pedal for about 15 seconds. There should be no pedal movement during this time. Pedal movement indicated a leak. Repeat the procedure using heavy pedal pressure (engine running for power brakes).
If there is pedal movement, but the fluid level is not low, the master cylinder has internal leakage. It must be rebuilt or replaced. If the fluid level is low, there is an external leak somewhere in the brake system. The leak must be repaired.
Depress the pedal and check for proper stop light operation.
To check power brake operation, depress and release the pedal several times while the engine is stopped. This eliminates vacuum from the system. Hold the brake down with moderate foot pressure and start the engine. IF the power unit is operating properly, the brake pedal moves downward when the engine is started.
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