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DIAGNOSIS AND TESTING - BASE BRAKE SYSTEM Base brake components consist of the brake shoes, calipers, rotors, brake lines, master cylinder, booster, and park- Brake diagnosis involves determining if the problem is related to a mechanical, hydraulic, or vacuum operated com- The first diagnosis step is the preliminary check. PRELIMINARY BRAKE CHECK 1. Check condition of tires and wheels. Damaged wheels and worn, damaged, or underinflated tires can cause pull, shudder, vibration, and a condition similar to grab. 2. If complaint was based on noise when braking, check suspension components. Jounce front and rear of vehicle and listen for noise that might be caused by loose, worn or damaged suspension or steering components. 3. Inspect brake fluid level and condition. Note that the brake reservoir fluid level will decrease in proportion to normal lining wear. Also note that brake fluid tends to darken over time. This is normal and should not be a. If fluid level is abnormally low, look for evidence of leaks at calipers, brake lines, and master cylinder. b. If fluid appears contaminated, drain out a sample to examine. System will have to be flushed if fluid is sep- arated into layers, or contains a substance other than brake fluid. The system seals and cups will also have 4. Check parking brake operation. Verify free movement and full release of cables and hand lever. Also note if vehicle was being operated with parking brake partially applied. 5. Check brake pedal operation. Verify that pedal does not bind and has adequate free play. If pedal lacks free play, check pedal and power booster for being loose or for bind condition. Do not road test until condition is corrected. 6. Check booster vacuum check valve and hose. 7. If components checked appear OK, road test the vehicle. ROAD TESTING 1. If complaint involved low brake pedal, pump pedal and note if it comes back up to normal height. 2. Check brake pedal response with transmission in Neutral and engine running. Pedal should remain firm under constant foot pressure. 3. During road test, make normal and firm brake stops in 25-40 mph range. Note faulty brake operation such as low pedal, hard pedal, fade, pedal pulsation, pull, grab, drag, noise, etc. 4. Attempt to stop the vehicle with the parking brake only and note grab, drag, noise, etc. PEDAL FALLS AWAY A brake pedal that falls away under steady foot pressure is generally the result of a system leak. The leak point Internal leakage (seal by-pass) in the master cylinder caused by worn or damaged piston cups, may also be the An internal leak in the ABS or junction block may also be the problem with no physical evidence. LOW PEDAL If a low pedal is experienced, pump the pedal several times. If the pedal comes back up worn linings or rotors are SPONGY PEDAL A spongy pedal is most often caused by air in the system. However, substandard brake lines and hoses can also 5 - 4 BRAKES - BASE - SERVICE INFORMATION KJ |