Hummer H1 (2006+). Manual - part 230

7-2

Brake System

______________________________________________________________

Hydro-Boost Replacement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-16

Speed Sensor Replacement   . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-62

Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-63
Removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-62

Tone Wheel Replacement   . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-64

POWER BRAKE SYSTEM DESCRIPTION 

The hydraulic power disc brake system is a four-wheel, inboard-mounted design. The dual reservoir master cylinder stores brake
fluid and converts mechanical brake pedal force to hydraulic pressure. The proportioning valve provides balanced front-to-rear
braking and activates the brake warning lamp in case of a brake hydraulic system malfunction. The dual reservoir master cylinder
provides fluid for separate front and rear brake systems (Figure  7-1). The hydro-boost provides power brake assist and is operated
by fluid pressure from the power steering pump. The hydro-boost is equipped with an accumulator. The accumulator stores nitro-
gen gas under pressure in the event that both the normal assist and accumulator assist are not available. The power steering pump
provides hydraulic oil pressure to operate the brake system’s hydro-boost feature. If the power steering pump fails to supply hy-
draulic pressure to the hydro-boost, the pressure stored in the accumulator will provide enough hydraulic pressure for approxi-
mately four power-assisted stops. Applying the parking brake prevents the rear brake rotors from rotating and can also be used to
help stop the vehicle in low speed emergency situations. 

The disc brakes are mounted on the output flanges of the front and rear axle assemblies.

Figure 7-1:   Brake System

BRAKE SYSTEM DIAGNOSIS

Road Testing

1.

If the red warning light is illuminated, note the pedal action and brake response.

2.

Check the brake pedal response with the transmission in Neutral and the engine running. The pedal should remain firm under
steady foot pressure. If the pedal falls away, problem is either in the hydro-boost, master cylinder, or brakeline.

3.

During a road test, make normal and firm brake stops in 25-40 mph range. Note if a faulty brake operation such as pull, grab,
drag, noise, fade, pedal pulsation, etc. (noise and pedal pulsation during an ABS event is normal).

4.

Inspect the suspect brake components and refer to problem diagnosis information for causes of various brake conditions.

Component Inspection

Fluid leak points and dragging brake units can usually be located without removing any components. The area around a leak point
will be wet with fluid. The components at a dragging brake unit will be quite warm or hot to the touch.

During component inspection, pay particular attention to heavily rusted/corroded brake components (e.g. rotors, caliper pistons,
lines, etc.).

BRAKE

BRAKE ROTORS

BRAKE CALIPERS

HYDRAULIC BRAKE 

LINES

CALIPERS

MASTER

CYLINDER

ABS

MODULATOR

6-1-05

_____________________________________________________________

 Brake System  7-3

5745804

Heavy accumulations of rust may be an indicator of rust and corrosion damage to a brake component. It is wise to remove surface
rust in order to accurately determine the depth of rust penetration and damage. Light surface rust is fairly normal and not a major
concern (as long as it is removed). However, heavy rust buildup, especially on high mileage vehicles, may actually cover structural
damage to such important components as brakelines and rotors.

Diagnosing Service Brake Problems

Brake Warning Light Operation

The red brake warning light will illuminate when the parking brakes are applied, and when there is a low fluid level in the brake
fluid reservoir. If the light comes on, first verify that the parking brakes are fully released. Then check the pedal action and fluid
level. If a problem is confirmed, inspect the wheel brake hydraulic system.

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. Internal leakage in the master cylinder caused by worn or damaged piston cups, may also be the prob-
lem cause.

If leakage is severe, fluid will be evident at or around the leaking component. However internal leakage in the master cylinder will
not be physically evident. Refer to the cylinder test procedure in this section.

Low Pedal

If a low pedal is experienced and the warning light is not on, worn lining and worn rotors are the most likely cause.

If the red warning light is on, low fluid in the master cylinder is the most likely cause. A leak at a caliper, brakeline, or brake hose
will cause the fluid level in the master cylinder reservoir to become low, triggering the low fluid switch and the red brake warning
light.

Spongy Pedal

A spongy pedal is most often caused by air in the system. However, substandard brake lines and hoses will also cause a condition
similar to a spongy pedal. The proper course of action is to bleed the system, or replace suspect quality brake lines and hoses.

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.

Brake Drag

Brake drag occurs when the lining is in constant contact with the rotor. 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, and rotors.

Brake drag also has a direct effect on fuel economy. If undetected, minor brake drag can be misdiagnosed as an engine or transmis-
sion/torque converter problem.

Minor drag will usually cause slight surface glazing of the lining. It can also generate hard spots in rotors from the overheat-cool
down process. In most cases, the rotors, 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 to the point of needing replacement.
The wheels, tires and brake components will be extremely hot. In severe cases, the lining may generate smoke as it chars from
overheating.

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

7-4

Brake System

______________________________________________________________

If brake drag occurs at all wheels, the problem may be related to a blocked master cylinder compensator port or a faulty hydro-
boost.

An improperly mounted or adjusted brakelight switch can prevent full brake pedal return. The result will be the same as if the cyl-
inder 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 deceleration 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 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 component are further causes of pull. A damaged front tire (bruised, ply separation)
can also cause pull.

Check the tires to be sure that they are inflated to the appropriate 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 combi-
nation 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 normally,
its braking effect is magnified. This causes pull to 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 al-
lowed 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 cylinder, or an im-
properly 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 materials 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 retains a normal appearance. Generally, the foreign material will remain
suspended in the fluid and be visible. The fluid and foreign 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 required. The master cyl-
inder will also have to be flushed or replaced if the contaminants cannot be removed. Foreign material lodged in the reservoir com-
pensator/return ports can cause brake drag by restricting fluid return after brake application.

Brake fluid contaminated by a non-recommended fluid, generally 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 contamination.

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.

_____________________________________________________________

 Brake System  7-5

5745804

The only real correction for contamination by non-recommended fluid is to flush the entire hydraulic system and replace all the
seals.

Brake Noise

Squeak/Squeal

The factory installed brakelining in HUMMER H1 vehicles is made from asbestos free materials. These materials have different
operating characteristics than previous lining materials. Under certain conditions, asbestos free lining may generate some squeak,
groan or chirp noise. This noise is considered normal and does not indicate a problem. The only time inspection is necessary, is
when noise becomes constant or when grinding, scraping noises occur.

Constant brake squeak or squeal may be due to linings that are wet or contaminated with brake fluid, grease, or oil. Glazed linings
and rotors with hard spots can also contribute to squeak. Dirt and foreign material embedded in the brake lining can also cause
squeak/squeal.

Loud brake squeak, squeal, scraping, or grinding sounds are a sign of severely worn brake lining. If the lining has worn completely
through in spots, metal-to-metal contact occurs. If the condition is allowed to continue, rotors can become so scored that replace-
ment is necessary.

Thump/Clunk

Thumping or clunk noises during braking are frequently not caused by brake components. In many cases, such noises are caused
by loose or damaged steering, suspension, drive line, or engine components. However, calipers that bind on the slide surfaces can
generate a thump or clunk noise. Loose adapter bolts or halfshaft-to-rotor bolts will cause noise. 

Chatter/Shudder

Brake chatter is usually caused by loose or worn components, or glazed/burnt lining. Rotors with hard spots can also contribute to
chatter. Additional causes of chatter are out-of-tolerance rotors, brake lining not securely attached to the shoes, loose wheel bear-
ings and contaminated brake lining.

Brake Lining Contamination

Brake lining contamination is a product of leaking calipers, driving through deep water puddles, or lining that has become covered
with grease or oil due to leaking axle seals.

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 little or no tread left can produce a grab-like condition as the tire loses and recovers traction.

Flat-spotted tires can cause vibration and wheel tramp and generate shudder during brake operation.

A tire with internal damage such as a severe bruise or ply separation can cause pull and vibration.

Diagnosing Parking Brake Problems

Parking Brake Problem Causes

In most cases, the actual cause of an improperly functioning parking brake (too loose/too tight/won’t hold), can be traced to a rear
brake component.

The leading cause of improper parking brake operation, is excessive clearance between the brake pads and the rotor surface. Exces-
sive clearance is a result of lining and/or rotor wear or inoperative adjuster components.

Inspect and adjust parking brake lever or linkage as necessary. 

BRAKE SYSTEM TROUBLESHOOTING

Parking Brake Does Not Hold Vehicle

1.

Check the parking brake adjustment and ensure the linkage and cables operate freely. Adjust the parking brake lever and/or 
cables or replace the damaged and worn parts.