Chevrolet Silverado / GMC Sierra. Service manual — part 2375

repeat steps 2-4.

9. If none of these conditions were found with the engine vacuum hose, then there is an engine

vacuum source problem, check the engine vacuum system.

10. Remove the vacuum brake booster check valve from the booster.

11. Install the check valve to the engine vacuum hose.

12. Install the vacuum gage to the check valve.

13. Start the engine and allow the engine to idle in PARK with the A/C system OFF, until

normal operating temperatures are reached.

14. Turn the ignition OFF.

15. Check to see if the engine vacuum reading is maintained within the specified normal engine

vacuum range.

Specification: 47-68 kPa (14-20 in Hg)

16. If the engine vacuum reading is maintained within the specified normal range, proceed to

step 18.

17. If the engine vacuum reading is NOT maintained within the specified normal range, replace

the brake booster check valve, then repeat steps 11-15.

18. Inspect the brake booster check valve grommet for the following conditions:

Loose connection to the vacuum brake booster

Deformation or contamination

Cracks, cuts, dry-rot

19. If any of these conditions were found with the check valve grommet, replace the grommet.

HYDRAULIC BRAKE ASSIST SYSTEM NOISE INSPECTION

Tools Required

J 28662 Brake Pedal Effort Gage. See Special Tools.

Inspection

1. Install the J 28662 to the brake pedal. See Special Tools.

2. Start the engine and allow it to idle. Ensure that the wheels are directed straight ahead.

3. Apply and maintain the brakes with 111 N (25 lb) of force to the brake pedal, as indicated

on the J 28662 . See Special Tools. Observe for noises.

4. Quickly release the brakes. Observe for noises.

2008 Chevrolet Silverado 1500

2008 BRAKES Hydraulic Brakes - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

5. If chatter and/or a vibration of the brake pedal was apparent during the brake system apply

of 111 N (25 lb) of force, the power steering pump drive belt not may have proper tension
or may be excessively worn.

6. If any significant noise was apparent during the quick release of the brakes following the

brake system apply of 111 N (25 lb) of force, there is likely a restriction in the hydraulic
brake booster power steering return hose.

7. Apply the brakes with 178 N (40 lb) of force to the brake pedal, as indicated on the J

28662 , then gradually increase pedal effort up to 445 N (100 lb) of force for not more than
5 seconds at a time. See Special Tools. Observe for noises.

8. Quickly release the brakes. Observe for noises.

9. If a hiss noise was apparent and increased during the brake system increasing apply of 178-

445 N (40-100 lb) of force, the hydraulic brake booster assist system is operating normally
from an audible standpoint.

10. If a clunk, chatter, and/or clicking noise was apparent during the quick release of the brakes

following the brake system increasing apply of 178-445 N (40-100 lb) of force, the
hydraulic brake booster assist system is operating normally from an audible standpoint.

BRAKE SYSTEM EXTERNAL LEAK INSPECTION

1. In order to inspect for external brake fluid leaks, first check the fluid level in the master

cylinder.

While a slight brake fluid level drop can be considered a normal condition due to brake
lining wear, a very low level may indicate a brake fluid leak in the hydraulic system.

2. If the fluid level is abnormally low, adjust the brake fluid level. Refer to Master Cylinder

Reservoir Filling.

3. Start the engine and allow it to idle.

IMPORTANT: Do NOT apply 445 N (100 lb) of force or more to the brake

system pedal for more than 5 seconds at a time.

CAUTION: Refer to Brake Fluid Irritant Caution .

NOTE:

Refer to Brake Fluid Effects on Paint and Electrical Components
Notice .

2008 Chevrolet Silverado 1500

2008 BRAKES Hydraulic Brakes - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

4. Apply constant, moderate foot pressure to the brake pedal.

If the brake pedal gradually falls away while under foot pressure, there may be a brake fluid
leak.

5. Turn OFF the ignition.

6. Visually inspect the following brake system components for brake fluid leaks, excessive

corrosion, and damage. Give particular attention to all brake pipe and flexible hose
connections to ensure that there are not any slight brake fluid leaks - even though the brake
pedal may feel firm and hold steady:

Master cylinder brake pipe fittings

All brake pipe connections

Brake pipes

Brake hoses and connections

Brake calipers and/or wheel cylinders, if equipped

7. While slight dampness around the master cylinder reservoir can be considered acceptable,

brake fluid leaking from any of the brake system components requires immediate attention.
If any of these components exhibit signs of brake fluid leakage, repair or replace those
components. After the repair or replacement, reinspect the hydraulic brake system to assure
proper function.

BRAKE SYSTEM INTERNAL LEAK TEST

1. Start the engine and allow it to idle.

2. Apply light, steady pressure to the brake pedal. Observe both the brake pedal feel and

travel.

3. Release the brakes and turn OFF the ignition.

4. If the brake pedal apply felt spongy, but the brake pedal travel was not excessive, perform

the following steps:

1. Inspect the brake system for external leaks. Refer to Brake System External Leak

Inspection .

CAUTION: Refer to Brake Fluid Irritant Caution .

NOTE:

Refer to Brake Fluid Effects on Paint and Electrical Components
Notice .

2008 Chevrolet Silverado 1500

2008 BRAKES Hydraulic Brakes - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

2. Pressure bleed the brake system in order to purge any air that may be trapped in the

system. Refer to Hydraulic Brake System Bleeding (Pressure)Hydraulic Brake
System Bleeding (Manual) .

Fig. 3: Cross Sectional View Of Brake Master Cylinder
Courtesy of GENERAL MOTORS CORP.

5. If the brake pedal apply did not feel spongy, but the brake pedal travel was excessive,

perform the following steps:

1. Loosen the master cylinder-to-brake power booster mounting nuts.

2. Carefully pull the master cylinder away from the brake power booster just enough to

inspect the mounting surface of the master cylinder.

3. Inspect the master cylinder mounting surface at the primary piston (2) for brake fluid

2008 Chevrolet Silverado 1500

2008 BRAKES Hydraulic Brakes - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado