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For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 1997 🢒 GMC P62 Commercial Chassis 🢒 Brakes 🢒 Hydraulic Brakes 🢒 Hydraulic Booster Diagnosis

Hydraulic Booster Diagnosis Gen.

Brake Pedal Returns Too Slowly

Checks

Action

Preliminary Checks
    • Check for excessive friction in the seals of the hydraulic booster. If necessary, overhaul with new seal kit. Refer to Seal Leak Diagnosis in Hydraulic Booster System Fluid Loss.
    • Check for faulty spool action. If necessary, flush the steering system while pumping the brake pedal. Refer to Flushing the Power Steering System in Power Steering.
    • Check for a restriction in the return hose from the hydraulic booster to the pump reservoir. If necessary, replace the hose. Refer to Power Steering Hoses in Power Steering.
    • Check for damage to the input rod end of the hydraulic booster. If necessary, replace the input rod and piston assembly.

Brake Pedal Vibrates

Checks

Action

DEFINITION: Brakes grab, hydraulic booster chatters, and brake pedal vibrates.

Preliminary Checks
    • Check for faulty spool action caused by contamination in the hydraulic system. If necessary, flush the steering system while pumping the brake pedal. Refer to Flushing the Power Steering System in Power Steering.
    • Check for a low level of fluid in the power steering pump. If necessary, fill the power steering reservoir and check for external leaks. Refer to Checking and Adding Power Steering Fluid in Power Steering.

Hydraulic Accumulator System Does Not Hold a Charge

Checks

Action

Preliminary Checks
    • Check for contamination in the power steering system or in the hydraulic booster system. If necessary, flush the steering system while pumping the brake pedal. Refer to Flushing the Power Steering System in Power Steering.
    • Check for internal leaks in the accumulator system. If necessary, overhaul the accumulator by using the accumulator rebuild kit and seal kit. Refer to the Light Duty Truck Unit Repair Manual.

Brakes Self-Apply When the Steering Wheel is Turned

Checks

Action

Preliminary Checks
    • Check for contamination in the power steering system or in the hydraulic booster system. If necessary, replace the hydraulic booster. Refer to Booster Assembly. Flush the hydraulic system. Refer to Flushing the Hydraulic Booster System.
    • Check for a restriction in the return hose from the hydraulic booster to the power steering pump reservoir. If necessary, replace the hose. Refer to Power Steering Hoses in Power Steering.

Hydraulic Booster Diagnosis BHB Noisy Brake Booster

Step

Action

Value(s)

Yes

No

1

Verify the complaint.

Is the noise normal

--

Go to Step 2

System OK

2

Does the noise occur during heavy braking and quick pedal release?

--

System OK

Refer to Normal Operational Noises

Go to Step 3

3
  1. Run the engine until it is at normal operating temperature.
  2. Operate the brakes with light pressure under normal driving conditions.

Is the noise present?

--

Go to Step 4

System OK

4

Refer to Noise Diagnosis of the Hydraulic Booster and perform any necessary repairs.

Has the abnormal noise been corrected?

--

System OK

--

Hydraulic Booster Diagnosis BHB Erratic Operation, Stick

Step

Action

Value(s)

Yes

No

1
  1. Run the engine at a fast idle.
  2. Pull the brake pedal rearward with approximately 44 N (100 lb) of force.
  3. Release the brake pedal and measure the distance to the floorboard.

Does the pedal return to its original position?

--

Go to Step 4

Go to Step 2

2
  1. Check the pedal to be sure it is operati--ng freely.
  2. Correct any sticking or binding as necessary.

Was any sticking or binding found?

--

Go to Step 3

Go to Step 4

3

Re-test the system.

Does the pedal return to its original position?

--

System OK

Go to Step 4

4

Check for an obstruction in the return line or for a kinked connection between the hydraulic booster and the reservoir of the power steering pump.

Is there an obstruction or kinked connection?

--

Go to Step 5

Go to Step 6

5
  1. Remove the obstruction or replace the line as necessary.
  2. Re-test the system.

Does the pedal return to its original position?

--

System OK

Go to Step 6

6

Check for a damaged reaction end.

Is the reaction end damaged?

--

Go to Step 7

Go to Step 8

7
  1. Repair or replace the hydraulic booster as necessary. Refer to Booster Assembly.
  2. Re-test the system.

Does the pedal return to its original position?

--

System OK

Go to Step 8

8
  1. Repair or replace the hydraulic booster or the master cylinder as necessary. Refer to Master Cylinder and Functional Test of the Hydraulic Booster System.
  2. Re-test the system.

Does the pedal return to its original position?

--

System OK

--

Hydraulic Booster Diagnosis BHB Excessive Brake Pedal Ef

Step

Action

Value(s)

Yes

No

1
  1. Engine OFF.
  2. Apply and release the brake pedal four times to deplete all hydraulic pressure from the hydraulic booster
  3. Apply the brake pedal and hold it with light pressure.
  4. While holding the brake pedal with light pressure, start the engine. If the power section is operating, the pedal will fall slightly and then hold. Less pressure will be needed to hold the brake pedal down to this position.

Is the power section operating?

--

Go to Step 2

Go to Step 7

2

Perform the leakdown test for the hydraulic booster accumulator. Refer to Leakdown Test for the Hydraulic Booster Accumulator.

Did the hydraulic booster pass the leakdown test?

--

Go to Step 3

Go to Step 4

3
  1. Thoroughly clean the hydraulic booster unit and all hose connections.
  2. Start the engine and run at idle speed.
  3. Apply 445 N (100 lb) of force to the brake pedal and hold. Do not hold brake pedal at this force for more than 5 seconds at a time.
  4. Check the hydraulic booster and hose connections for leaks.

Were there any leaks?

--

Go to Step 4

Go to Step 5

4

Repair or replace the hoses or hydraulic booster as necessary. Refer to Booster Assembly.

Have repairs been completed?

--

Go to Step 5

--

5
  1. Apply and release the brake pedal several times.
  2. Apply and hold the brake pedal with a force of about 110-155 N (25-35 lb).

Does the pedal fall away?

--

Go to Step 6

System OK

6
  1. Repair leaks elsewhere in the system. Refer to Brake Fluid Leaks.
  2. Re-test the system.

Is the system functioning normally?

System OK

--

7

Check the fluid level in the reservoir of the power steering pump.

Is the fluid level OK?

--

Go to Step 8

Go to Step 15

8

Check the condition and tension of the drive belt for the power steering pump.

Is the belt loose or damaged?

Go to Step 9

Go to Step 10

9
  1. Adjust or replace the drive belt as necessary. Refer to [A SECTION].
  2. Re-test the system.

Is the system operating normally?

--

System OK

Go to Step 10

10

Is the power steering pump running slowly?

Go to Step 11

Go to Step 12

11
  1. Adjust the power steering pump.
  2. Re-test the system.

Is the system operating normally?

System OK

Go to Step 12

12

Perform the Power Steering System Test. Refer to Power Steering System Test.

Is the output of the power steering pump within specification?

--

Go to Step 14

Go to Step 13

13
  1. Replace the power steering pump.
  2. Re-test the system.

Is the system operating normally?

--

System OK

Go to Step 14

14
  1. Repair or replace the hydraulic booster as necessary. Refer to Booster Assembly.
  2. Re-test the system.

Is the system operating normally?

System OK

--

15
  1. Add power steering fluid.
  2. Re-test the system.

Is the system operating normally?

System OK

Go to Step 16

16
  1. Perform the Fluid Leak Check for the power steering system. Refer to Fluid Leak Check in Power Steering.
  2. Re-test the system.

Is the system operating normally?

--

System OK

Go to Step 17

17
  1. Thoroughly clean the hydraulic booster and all hose connections.
  2. Start the engine and run at idle speed.
  3. Apply the brake pedal with about 445 N (100 lb) of force.
  4. Check the hydraulic booster and hose fittings for leaks.
  5. Repair hose fittings as necessary.

Does the hydraulic booster leak?

Go to Step 18

Go to Step 19

18

Repair or replace the hydraulic booster as necessary.

Have the repairs been completed?

Go to Step 19

--

19

Re-test the system.

Is the system operating normally?

--

System OK

--