Changes in brake pedal feel or travel signal something could be wrong in brake system. When diagnosing brake system problems, use brake warning light, pedal feel/travel and fluid level as indicators. Following symptoms indicate brake trouble:
Pedal goes down fast.
Check for leaks or air in system.
Pedal goes down slowly.
Check for external or internal leaks.
Pedal is low and/or feels spongy.
Check for empty master cylinder reservoir, reservoir cap vent holes clogged, rear brakes out of adjustment or air in system.
Pedal effort too high.
Check for binding or obstruction in pedal linkage. Check for poor booster assist.
Brake warning light is on.
Check for low fluid level, ignition wires too close to fluid level indicator assembly, damaged indicator float, or applied parking brake.
POWER BRAKE FUNCTION TEST
With engine stopped, remove vacuum in system by pumping brake pedal several times. Push pedal down as far as it will go. If pedal moves downward slowly, hydraulic system is leaking. Check hydraulic system for leaks.
With pedal pushed down as far as it will go, start engine. If pedal moves downward, vacuum system is okay. If pedal does not change position, a problem exists in vacuum system. Check vacuum system for leaks.
BRAKE WARNING LIGHT
Brake warning light should only come on when ignition is in START position or when ignition is on with parking brake applied or fluid level low.
If brake warning light does not come on when brake fluid is low, manually push reservoir float to bottom of reservoir. If light still does not come on, check fuse, wiring and bulb. Repair as necessary. If bulb and related circuitry are okay, replace reservoir assembly.
If brake warning light does not come on when parking brake is applied, check parking brake switch, wiring and bulb. Repair as necessary. With parking brake released and master cylinder reservoir full, turn ignition on. If warning light is on, check for shorted, grounded or defective warning switches or wiring. Repair as necessary. Turn ignition switch to Start position. If brake warning light does not come on as a bulb check function, check fuse, bulb and wiring. Repair as necessary.
BRAKE PEDAL RESERVE
If complaint is low or bottoming out brake pedal, run engine at idle with transmission in Park or Neutral. Lightly depress brake pedal 3 or 4 times. Wait 15 seconds for vacuum to build in booster. Depress brake pedal until it stops moving downward.
While holding pedal down, raise engine speed to about 2000 RPM. Release accelerator pedal. Brake pedal should move downward as engine speed returns to idle. If results are correct, system has proper pedal reserve. If results are not correct, check for adequate vacuum. If vacuum is okay, replace vacuum booster.
VACUUM POWER BOOSTER
With a "T" fitting, connect vacuum gauge into vacuum line between engine and power brake booster. With engine at operating temperature, gauge should read 15-19 in. Hg vacuum at idle with transmission in Neutral. If reading is less than specification, stop engine, disconnect vacuum hose at power brake booster and cap open end of hose and open port of vacuum "T".
Start engine, and allow it to idle. If reading is still less than 15-19 in. Hg vacuum, engine is producing low vacuum and mechanical problem must be corrected. If vacuum is to specification, check plastic check valve, rubber grommet and vacuum hose connection at power brake booster.
With low engine vacuum corrected and/or leaking components replaced, start engine, and allow it to idle. Stop engine, and depress brake pedal for a few seconds. If vacuum drops to zero, booster is leaking and requires replacement.
VACUUM BOOSTER CHECK VALVE
Remove vacuum booster hose from power brake booster and check valve connection. Manifold vacuum should be available at check valve end of vacuum booster hose with engine idling and transmission in Neutral.
Operate engine a minimum of 10 seconds at fast idle. Stop engine, and let vehicle stand for 10 minutes. Apply brake pedal with about 20 lbs. (9 kg) of force. Brake pedal feel should be same as that noted with engine operating. If brake pedal feels hard, replace check valve and retest.
Friday, December 19th, 2008 AT 9:25 PM