Dodge RAM TRUCK 1500 1995 Service Manual page 177

B R
SERVICE BRAKE DIAGNOSIS 5 - 5
HARD PEDAL OR HIGH PEDAL EFFORT
A hard pedal can be caused by a faulty power
booster, booster check valve, or binding pedal or
booster. High pedal effort is usually due to lining
that is water soaked, contaminated, glazed, or badly
worn.
On diesel engine models, high pedal effort may be
the result of a low vacuum condition. If the booster
and check valve are OK, the problem may be related
to a vacuum pump hose, hose connection, hose fit­
ting, pump diaphragm, or drive gear.
Vacuum pump output can be checked with a stan­
dard vacuum gauge. Vacuum output should range
from 8.5 to 25 inches vacuum. If vacuum pump out­
put is within limits, check the power booster and
check valve as described in this section.
BRAKE DRAI
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. It is a
product of incomplete brakeshoe release. Drag can be
minor, or severe enough to overheat the linings, ro­
tors and drums.
Brake drag can also effect fuel economy. If undetec­
ted, minor brake drag can be misdiagnosed as an en­
gine, torque converter, or transmission problem.
Minor drag will usually cause slight surface char­
ring of the lining. It can also generate hard spots in
rotors and drums from the overheat-cool down pro­
cess. 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 se­
vere cases, the lining may generate smoke as it chars
from overheating.
Some 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 bracket
• drum brakeshoes binding on worn or damaged
support plates
• misassembled components
• adjuster screws reversed
If brake drag occurs at all wheels, the problem may
be related to a blocked master cylinder return port,
or faulty power booster that binds and does not re­
lease.
BRAKE FADE
Brake fade is a product of overheating caused by
brake drag. However, overheating and subsequent
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. Refer to the Brake Drag information in this
section for additional causes.
BRAKE PEDAL PULSATION
Pedal pulsation in RWAL equipped models, is
caused by brake parts that are loose, or out of toler­
ance limits. This type of pulsation is experienced ev­
ery time the brakes are applied.
Disc brake rotors with excessive lateral runout or
thickness variation, or out of round brake drums are
the primary causes of pulsation.
On vehicles with ABS brakes, remember that pedal
pulsation is normal during antilock mode brake
stops. If pulsation occurs during light to moderate
brake stops, a standard brake part is either loose, or
worn beyond tolerance.
BRAKE PULL
A front pull condition could be the result of contam­
inated lining in one caliper, seized caliper piston,
binding caliper, loose caliper, loose or corroded slide
pins, improper brakeshoes, or a damaged rotor.
A worn, damaged wheel bearing or suspension com­
ponent are further causes of pull. A damaged front
tire (bruised, ply separation) can also cause pull.
A common and frequently misdiagnosed pull condi­
tion is where direction of pull changes after a few
stops. The cause is a combination of brake drag fol­
lowed by fade at one of the brake units.
As the dragging brake overheats, efficiency is so re­
duced that fade occurs. Since the opposite brake unit
is still functioning normally, its braking effect is
magnified. This causes pull to switch direction in fa­
vor of the normally functioning brake unit.
When diagnosing a change in pull condition, re­
member 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 an improp­
erly adjusted or seized parking brake cable, contam­
inated lining, bent or binding shoes and support
plates, or improperly assembled components. This is
particularly true when only one rear wheel is in­
volved. However, when both rear wheels are affected,
the master cylinder or proportioning valve could be
at fault.