Hummer H1 Service Manual page 442

_____________________________________________________________
®
Some common causes of brake drag are:
• seized or sticking caliper piston
• caliper binding on bushings or pin slides
• incorrect length caliper mounting bolts (too long)
• loose caliper mounting bracket
• misassembled components
• misadjusted brakelight switch
• binding brake pedal
• master cylinder/hydroboost internal fault
• improperly adjusted parking brake
If brake drag occurs at all wheels, the problem may be related
to a blocked master cylinder compensator port or faulty hydro-
boost.
An improperly mounted or adjusted brakelight switch can pre-
vent full brake pedal return. The result will be the same as if
the cylinder compensator ports are blocked. In this case, the
brakes would be partially applied all the time causing drag.
Brake Fade
Brake fade is a product of overheating caused by brake drag.
However, brake overheating and subsequent fade can also be
caused by riding the brake pedal, making repeating high decel-
eration stops in a short time span, or constant braking on steep
roads. Refer to the Brake Drag information in this section for
causes.
Pedal Pulsation
Pedal pulsation whenever the ABS is not active, is caused by
components that are loose, or beyond tolerance limits.
Disc brake rotors with excessive lateral runout or thickness
variation are the primary causes of pulsation. Other causes are
loose calipers, and worn, damaged tires.
Brake Pull
A front pull condition could be the result of contaminated lin-
ing 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 component are
further causes of pull. A damaged front tire (bruised, ply sepa-
ration) can also cause pull.
Check the tires to be sure that they are inflated to the appropri-
ate tire pressure. It could be another cause of your vehicle
"pulling" to one side.
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 the dragging
brake unit.
As the dragging brake overheats, efficiency is so reduced that
fade occurs. If the opposite brake unit is still functioning nor-
mally, its braking effect is magnified. This causes pull to
05745159
switch direction in favor of the brake unit that is functioning
normally.
When diagnosing a change in pull condition, 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
Rear grab (or pull) is usually caused by contaminated lining,
bent or binding pads or improperly assembled components.
This is particularly true when only one rear wheel is involved.
However, when both rear wheels are affected, the master cylin-
der, or an improperly adjusted parking brake 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
lightly applied for a mile or two. However, if the lining is both
wet and dirty, disassembly and cleaning will be necessary.
Brake Fluid Contamination
There are two basic causes of brake fluid contamination. The
first involves allowing dirt, debris, water, or other liquid mate-
rials to enter the cylinder reservoirs when the cover is off. The
second involves topping off, or filling the cylinder reservoirs
with a non-recommended fluid.
Brake fluid contaminated with only dirt or debris usually re-
tains a normal appearance. Generally, the foreign material will
remain suspended in the fluid and be visible. The fluid and for-
eign material can be removed from the reservoir with a suction
gun but only if the brakes have not been applied. If the brakes
are applied after contamination, system flushing will be re-
quired. The master cylinder will also have to be flushed or re-
placed if the contaminants cannot be removed. Foreign
material lodged in the reservoir compensator/return ports can
cause brake drag by restricting fluid return after brake applica-
tion.
Brake fluid contaminated by a non-recommended fluid, gener-
ally appears highly discolored, milky, oily looking, or foamy.
In some cases, it may even appear as if the fluid contains
sludge. However, be advised that brake fluid will darken
over time and occasionally be cloudy in appearance. These
are normal conditions and should not be mistaken for con-
tamination.
If some type of oil has been added to the system, the fluid will
separate into distinct layers. To verify this, drain off a sample
with a clean suction gun. Then pour the sample into a glass
container and observe fluid action. If the fluid separates into
distinct layers, it is definitely contaminated.
The only real correction for contamination by non-recom-
mended fluid is to flush the entire hydraulic system and replace
all the seals.
Brake System 7-3