Here's the entire procedure:
Road Testing
If complaint involved low brake pedal, pump pedal and note if it comes
back up to normal height.
Check brake pedal response with transmission in Neutral and engine
running. Pedal should remain firm under constant foot pressure.
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.
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 could be at a brake line,
fitting, hose, or caliper/wheel cylinder. If leakage is severe, fluid
will be evident at or around the leaking component.
Internal leakage (seal by-pass) in the master cylinder caused by worn or
damaged piston cups, may also be the problem cause.
An internal leak in the ABS or RWAL system 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, rotors, drums, or rear brakes out of
adjustment are the most likely causes. The proper course of action is to
inspect and replace all worn component and make the proper adjustments.
SPONGY PEDAL
A spongy pedal is most often caused by air in the system. However, thin
brake drums or substandard brake lines and hoses can also cause a spongy
pedal. The proper course of action is to bleed the system, and replace
thin drums and substandard quality brake hoses if suspected.
HARD PEDAL OR HIGH PEDAL EFFORT
A hard pedal or high pedal effort may be due to lining that is water
soaked, contaminated, glazed, or badly worn. The power booster or check
valve could also be faulty.
PEDAL PULSATION
Pedal pulsation is caused by components that are loose, or beyond
tolerance limits.
The primary cause of pulsation are disc brake rotors with excessive
lateral runout or thickness variation, or out of round brake drums.
Other causes are loose wheel bearings or calipers and worn, damaged
tires.
NOTE: Some pedal pulsation may be felt during ABS activation.
BRAKE DRAG
Brake drag occurs when the lining is in constant contact with the rotor
or drum. Drag can occur at one wheel, all wheels, fronts only, or rears
only.
Drag is a product of incomplete brake shoe release. Drag can be minor or
severe enough to overheat the linings, rotors and drums.
Minor drag will usually cause slight surface charring of the lining. It
can also generate hard spots in rotors and drums from the overheat-cool
down process. In most cases, the rotors, drums, wheels and tires are
quite warm to the touch after the vehicle is stopped.
Severe drag can char the brake lining all the way through. It can also
distort and score rotors and drums to the point of replacement. The
wheels, tires and brake components will be extremely hot. In severe
cases, the lining may generate smoke as it chars from overheating.
Common causes of brake drag are:
Seized or improperly adjusted parking brake cables.
Loose/worn wheel bearing.
Seized caliper or wheel cylinder piston.
Caliper binding on corroded bushings or rusted slide surfaces.
Loose caliper mounting.
Drum brake shoes binding on worn/damaged support plates.
Mis-assembled components.
Long booster output rod.
If brake drag occurs at all wheels, the problem may be related to a
blocked master cylinder return port, or faulty power booster (binds-does
not release).
BRAKE FADE
Brake fade is usually a product of overheating caused by brake drag.
However, brake overheating and resulting fade can also be caused by
riding the brake pedal, making repeated high deceleration stops in a
short time span, or constant braking on steep mountain roads.
BRAKE PULL
Front brake pull condition could result from:
Contaminated lining in one caliper
Seized caliper piston
Binding caliper
Loose caliper
Rusty caliper slide surfaces
Improper brake shoes
Damaged rotor
A worn, damaged wheel bearing or suspension component are further causes
of pull. A damaged front tire (bruised, ply separation) can also cause
pull.
A common and frequently misdiagnosed pull condition is where direction
of pull changes after a few stops. The cause is a combination of brake
drag followed by fade at one of the brake units.
As the dragging brake overheats, efficiency is so reduced that fade
occurs. Since the opposite brake unit is still functioning normally, its
braking effect is magnified. This causes pull to switch direction in
favor of the normally functioning brake unit.
An additional point when diagnosing a change in pull condition concerns
brake cool down. Remember that pull will return to the original
direction, if the dragging brake unit is allowed to cool down (and is
not seriously damaged).
REAR BRAKE GRAB OR PULL
Rear grab or pull is usually caused by improperly adjusted or seized
parking brake cables, contaminated lining, bent or binding shoes and
support plates, or improperly assembled components. This is particularly
true when only one rear wheel is involved. However, when both rear
wheels are affected, the master cylinder or proportioning valve could be
at fault.
BRAKES DO NOT HOLD AFTER DRIVING THROUGH DEEP WATER PUDDLES
This condition is generally caused by water soaked lining. If the lining
is only wet, it can be dried by driving with the brakes very lightly
applied for a mile or two. However, if the lining is both soaked and
dirt contaminated, cleaning and/or replacement will be necessary.
BRAKE LINING CONTAMINATION
Brake lining contamination is mostly a product of leaking calipers or
wheel cylinders, worn seals, driving through deep water puddles, or
lining that has become covered with grease and grit during repair.
Contaminated lining should be replaced to avoid further brake problems.
WHEEL AND TIRE PROBLEMS
Some conditions attributed to brake components may actually be caused by
a wheel or tire problem.
A damaged wheel can cause shudder, vibration and pull. A worn or damaged
tire can also cause pull.
Severely worn tires with very little tread left can produce a grab-like
condition as the tire loses and recovers traction. Flat-spotted tires
can cause vibration and generate shudder during brake operation. A tire
with internal damage such as a severe bruise, cut, or ply separation can
cause pull and vibration.
- Bernd
This archive was generated by hypermail 2b29 : Fri Jun 20 2003 - 12:02:03 EDT