MASTER CYLINDER
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, low vacuum (diesel) or applied parking brake.
HYDRO-BOOST UNIT
Use following list of symptoms to aid in diagnosing Hydro-Boost problems:
Brake pedal returns slowly:
Check for restriction in return line between Hydro-Boost and power steering reservoir.
Check for incorrectly connected return line.
Reposition brake pedal or add return spring.
Replace Hydro-Boost.
Brakes grab:
Tighten power steering belt.
Flush steering system while pumping brake pedal.
Replace Hydro-Boost.
Hydro-Boost chatter/Pedal vibration:
Tighten power steering belt.
Check for low fluid level.
Accumulator leaks down/No reserve brake application:
Replace Hydro-Boost.
High brake pedal effort:
Tighten or replace power steering belt.
Low fluid level.
Replace Hydro-Boost.
Brakes apply by themselves:
Restriction in return line.
Return line not connected correctly.
Replace Hydro-Boost.
TESTING
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.
MASTER CYLINDER
To check master cylinder for internal leak, disconnect brake lines at master cylinder. Plug outlet ports of master cylinder. Apply brakes. If pedal height cannot be maintained, master cylinder has internal leak and must be rebuilt or replaced.
BRAKE WARNING LIGHT
Brake warning light should only come on when ignition switch is in START position or when ignition is on with parking brake applied or fluid level low. On diesel vehicles, brake warning light should also come on when vacuum is 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 brake pedal is low or bottoming out, 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, increase 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.
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Saturday, December 20th, 2008 AT 10:30 AM
