Hummer H1 (2006+). Manual - part 231

7-6

Brake System

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2.

Inspect the rear brake caliper brake pads for serviceability. The minimum brake lining thickness is 1/8 in. (3.2 mm). Replace
all the rear brake pads if any of the pads does not meet the thickness specifications.

3.

Check the calipers for binding or dragging. Check for binding caliper guide pins. 

Low or Spongy Brake Pedal on First Application or Pedal Goes to Floor

CAUTION: The HUMMER H1 is equipped with DOT 3 brake fluid. Do not mix with other brake fluids. Failure to use the proper
brake fluid will damage the brake system.

WARNING:  Always wear eye protection when bleeding brakes. Failure to do this may cause personal injury if
brake fluid comes in contact with eyes.

1.

Remove the master cylinder cover and visually check the reservoirs for low fluid level or contamination.

a.

If the fluid is contaminated, flush the system with clean brake fluid.

b.

If the fluid is low, check for worn brake pads, observe for leakage, broken, cracked or kinked lines, worn master cylinder,
etc. Replace any worn parts. Add DOT 3 as needed.

2.

Check the hydro-boost. Depress the brake pedal several times, with the engine off, to exhaust the accumulator pressure.
Depress the brake pedal and start the engine.

a.

The brake pedal should fall, then push back against the operator's foot.

b.

Perform a pressure test (Section 8). 

c.

Replace the hydro-boost if is not operating properly.

3.

Bleed the master cylinder then bleed the brakes.

Decreased Brake Pedal Travel or Slow Return

1.

Check for worn brake pedal return spring. Replace the spring if worn.

2.

Check the brake pedal bushings for signs of wear or binding. Replace the pedal bushings if worn and lubricate as needed.

3.

Check for kinked or damaged brake lines which may restrict the brake fluid. Replace any damaged lines.

4.

Check the hydro-boost. Depress the brake pedal several times, with the engine off, to exhaust the accumulator pressure.
Depress the brake pedal and start the engine.

a.

The brake pedal should fall, then push back against the operator's foot.

b.

Perform a pressure test (Section 8).

c.

Replace the hydro-boost if is not operating properly.

5.

Check the brake calipers for binding as a result of corrosion or dirt. Check the brake rotors for free movement. If the rotors do
not move freely, remove the calipers and clean the caliper guide pins.

6.

Check the parking brake cable for proper operation and adjustment. Repair as required.    

Excessive Pedal Pressure Required to Stop Vehicle

1.

Remove the master cylinder cover and visually check the reservoir for low fluid level or contamination.

a.

If the fluid is contaminated, flush the system with clean brake fluid.

b.

If the fluid is low, check for worn brake pads, observe for leakage, broken, cracked or kinked lines, worn master cylinder,
etc. Replace any worn parts. 

2.

Check the fluid in the power steering pump reservoir. Fill the fluid to the proper level (Section 1).

3.

Check the serpentine belt tension. Replace the belt (Section 8) if necessary.

4.

Check the brake pads for proper installation, contamination, or distortion. Check brake pads for excessive wear. Minimum
brake lining thickness is 1/8 in. (3.2 mm). Replace the brake pads as axle sets (front or rear) if any pad does not meet
specifications

NOTE: To preserve even braking, both calipers must be in equal condition.

5.

Check the brake calipers for binding as a result of corrosion or dirt. Check the brake rotors for free movement. If the rotors do
not move freely, remove the calipers and clean the caliper guide pins.

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 Brake System  7-7

5745804

6.

Check for frozen piston in the brake caliper. If the inner pad is not worn to the limit, but the piston cannot be retracted, rebuild
or replace both calipers.

7.

Check for pinched or kinked supply and return lines to the hydro-boost. Reposition or replace any damaged lines.

8.

Check for damaged brake lines. Replace any damaged brake lines found.

9.

Check for malfunctioning hydro-boost. Depress the brake pedal several times, with the engine off, to exhaust the accumulator
pressure. Depress the brake pedal and start the engine.

a.

The brake pedal should fall, then push back against the operator's foot.

b.

Perform a pressure test (Section 8). 

c.

Replace the hydro-boost if it is not operating properly.

10. Check the power steering system. Refer to step 6 in the Diagnostic portion of Hard Steering in Section 8.

Noisy Brakes

1.

Check the brake pads for proper installation, contamination, or distortion. Check brake pads for excessive wear. Minimum 
brake lining thickness is 1/8 in. (3.2 mm). Replace the brake pads as sets (front or rear) if any of the pads does not meet speci-
fications. Make sure pads are flat and smooth.

2.

Check the rotors for glazing or scoring. Turn the rotor if glazed or scored. Do not exceed the minimum thickness shown on the
inside of the rotor hat section. It is not recommended that the rotors be turned when spotted or heat checked.

3.

 Check the halfshaft mounting.

a.

Apply a thread-locking compound to the halfshaft and rotor-to-output flange capscrews, or use bolts with pre-applied
compound, and torque to 57 lb-ft (77 N•m).

b.

Apply a thread-locking compound to the halfshaft retaining capscrew (in geared hub) and torque to 40 lb-ft (54 N•m).
Tighten any loose fasteners.

Brake Chatter

On some new vehicles, roughness or a chatter sound from the brakes may be noticed during low speed brake application. The noise
is a result of the lining edges of an unburnished brake pad rubbing against the rotor. Burnishing is a part of the vehicle break-in pro-
cess which fully seats and conditions new brake pads. Although annoying, the brake noise is not detrimental to vehicle safety or
performance, and will eventually be eliminated through normal brake use.

If chatter or roughness persists on a new vehicle, then chamfering of the brake pad lining edges can be performed. Chamfering of
the brake pad is done by slightly grinding or filing the edge of the pad lining on a grinding wheel.

To complete the chamfering procedure:

1.

Remove the eight service brake pads from the vehicle. Mark each pad for vehicle and caliper location. 

2.

Chamfer (grind or file) the brake pad lining edges. Ensure both brake pad lining edges are chamfered on each of the eight
brake pads (Figure  7-2). Make sure pads are flat and even.

3.

Install the eight service brake pads at the original vehicle and caliper location.

4.

Operate the vehicle and check the brakes for proper operation.

Figure 7-2:   Chamfering Brake Pad 

BRAKE PAD LINING EDGE

AREA TO BE REMOVED BY

BRAKE PAD

LINING EDGE

CHAMFERING (GRINDING)

VIEW DEPICTS BRAKE PAD 

AFTER BEING CHAMFERED

18°

1/4”

SIDE VIEW OF BRAKE

PAD LINING EDGE

S07-043

7-8

Brake System

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Booster or Pedal Pulsation

1.

Check the hydro-boost. Depress the brake pedal several times, with the engine off, to exhaust accumulator pressure. Depress 
the brake pedal and start the engine.

a.

Brake pedal should fall, then push back against the operator's foot.

b.

Perform a pressure test (Section 8). 

c.

Replace the hydro-boost if it is not operating properly.

2.

Check thr halfshaft and rotor mounting for missing or loose capscrews. Replace the inclined-cam washers, apply a thread
locking compound and torque capscrews to 57 lb-ft (77 N•m).

3.

Check the brake rotor lateral run-out. Refinish any rotor not meeting the specifications. Refer to Checking Lateral Runout in
this section for more information.

Erratic Braking Action

1.

Check for the correct tire pressure. 

2.

Check the brake pads for excessive wear. Minimum brake lining thickness is 1/8 in. (3.2 mm). Replace the brake pads as sets
(front or rear) if any pad does not meet the specifications.

3.

Check the brake calipers for binding as a result of corrosion or dirt. Check for seized or binding brake caliper pistons. Repair
any binding or seized caliper pistons. Check the brake rotors for free movement. If the rotors do not move freely, remove the
calipers and clean caliper guide pins  (Figure  7-3).

NOTE: Calipers pins must be replaced in pairs. Caliper pins and bushing should be lubricated with an approved brake component
lubricant.

4.

Check for leaking caliper piston seals. Replace or rebuild any calipers with leaking seals. 

5.

Check the rotor for glazing or scoring. Turn the rotor if glazed or scored. Do not exceed the minimum thickness shown on the
inside of the rotor hat section. It is not recommended that the rotors be turned when spotted or heat checked.

6.

Check for damaged brake lines. Replace any damaged brake lines.

7.

Check the ABS operation. Check the ABS warning lamp. Perform a 20 mph hard deceleration on a slippery surface (sand,
gravel, snow, etc). If one or more of the wheels lock repair the ABS. Refer to ABS diagnostics in this secion.

8.

Check the toe adjustment. Adjust the toe, if necessary. Refer to Section 5.

Figure 7-3:   Brake Caliper and Brake Pads

COOLING

FINS

CALIPER

PINS

CALIPER

COUPLING

BRAKE

BRAKE PAD

ROTOR

GUIDE

LINE/HOSE

AND COPPER

WASHER

BUSHING

AND

SLEEVE

BRAKE

PAD

ADAPTER

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 Brake System  7-9

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HYDRO-BOOST SYSTEM DIAGNOSIS

Noise Diagnosis

The following noises are associated with the hydro-boost and may or may not be cause for concern. Some noises are normal and
for the most part temporary in nature. Other noises may be a sign of excessive wear or the presence of air in either the booster or
the steering system. 

1.

A moan or low frequency hum usually accompanied by a vibration in the pedal or steering column may be observed during 
parking maneuvers or other low-speed maneuvers. This may be caused by a low fluid level in the power steering pump or by 
air in the fluid. Holding the pump at relief pressure (steering wheel held all the way in one direction) for more than five sec-
onds will cause air to enter the system. Check the fluid level and fill if needed. The system must then sit for one hour to remove
the air. 

2.

A high-speed fluid noise may be heard when the brake pedal is fully depressed. This condition is normal.

3.

Whenever the accumulator pressure is used, a slight hiss may be noticed. It is the sound of the hydraulic fluid escaping through
the accumulator valve, and is completely normal. 

4.

After the accumulator has been emptied and the engine is started again, another hissing sound may be heard during the first
brake application or the first steering maneuver. This is caused by the fluid rushing through the accumulator charging orifice.
It is normal and will only be heard once after the accumulator is emptied. If this sound continues however, even though no
apparent accumulator pressure assist was made, it could be an indication that the accumulator is not holding pressure and
should be checked using the procedure Accumulator Leakdown Test in this section. 

Table 1: 

PROBLEM

POSSIBLE CAUSE

CORRECTION

Slow Brake Pedal 
Return
or
Brakes Apply When 
Turning Steering 
Wheel

1.

Damaged/broken return spring.

2.

Excessive seal friction in booster. 

3.

Faulty spool action. 

4.

Restriction in return line from booster to 
pump reservoir. 

5.

Damaged input rod end.

1.

Replace spring.

2.

Replace the hydro-boost. 

3.

Flush the steering system while pumping the 
brake pedal. 

4.

Replace the line. 

5.

Replace the hydro-booster. 

Grabs Suddenly
or
Booster Chatters - 
Pedal Vibrates

1.

Faulty spool action caused by contami-
nation.

2.

Power steering pump belt slips. 

3.

Low fluid level in power steering pump. 

1.

Flush steering system while pumping brake pedal.

2.

Replace tensioner and/or belt.

3.

Fill reservoir and check for external leaks.

Accumulator
Leak-Down 
Reserve System 
Does Not Hold 
Charge

1.

Contamination in steering hydro-boost 
system.

2.

Internal leakage in accumulator system

1.

Flush steering system while pumping brake pedal.

2.

Replace hydro-boost.

Excessive Brake 
Pedal Efforts

1.

Loose, glazed, or broken pump belt.

2.

No fluid in pump reservoir.

3.

Leaks in system hoses.

4.

Leaks at tube fittings and connections.

5.

Leakage at pneumatic accumulator seal.

6.

Leakage at piston seal.

7.

Leakage at input seal.

8.

Leakage at cover-to-housing seal.

9.

Leakage at spool plug seal.

10. Leakage at ball plug

1.

Replace tensioner and/or replace belt.

2.

Fill reservoir and check for external leaks.

3.

Replace faulty parts.

4.

Tighten fittings or replace tube seats or O-rings.

5.

Replace O-ring.

6.

Overhaul with new seal kit.

7.

Overhaul with new seal kit.

8.

Overhaul with new seal kit.

9.

Overhaul with spool plug seal kit.

10. Replace hydro-boost.

NOTE: The power steering fluid and brake fluid cannot be mixed. If the brake seals contact steering fluid or the steering seals
contact brake fluid, seal damage will result.