DEFINITION: This diagnostic table is designed to diagnose ONLY the components of the brake ASSIST system in order to determine if the brake ASSIST system is operating properly. You will be directed by the appropriate Symptom Table to go to other brake system diagnostic tables as appropriate.

Were you sent here from a Brake Symptom Table?

Go to Step 2

Go to Diagnostic Starting Point - Hydraulic Brakes

Is the symptom related to brake system noise?

Go to Step 4

Go to Step 3

Is the symptom related to the brake system self-applying when the steering wheel is being turned?

Go to Step 14

Go to Step 9

Check the hydraulic assist system operation for the presence of abnormal noise. Refer to Hydraulic Brake Assist System Noise Inspection .

Did the noise inspection indicate a concern with the power steering system drive belt?

Go to Step 5

Go to Step 6

Replace the power steering system drive belt. Refer to Drive Belt Replacement in Engine Mechanical - 8.1L.

Did you complete the replacement?

Go to Step 6

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Did the noise inspection indicate a possible restriction in the hydraulic brake booster power steering system return hose?

Go to Step 7

Go to Step 8

Replace the restricted hydraulic brake booster power steering return hose. Refer to Power Brake Booster Outlet Hose Replacement in Power Steering System.

Did you complete the replacement?

Go to Step 8

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During the noise inspection, did the hydraulic brake booster produce abnormal noises?

Go to Step 14

Go to Step 9

Inspect for proper brake pedal travel. Refer to Brake Pedal Travel Measurement and Inspection .

Is the brake pedal travel distance within the acceptable limits?

Go to Step 12

Go to Step 10

1. Inspect for worn, missing, misaligned, bent or damaged brake pedal system components.

•	For the brake pedal pushrod component inspection, refer to Brake Pedal Pushrod Inspection .

•	Inspect the brake pedal bushings for excessive wear and/or damage and inspect the brake pedal for a misaligned, bent and/or damaged condition.

2. Replace the brake pedal system components that are worn, missing, misaligned, bent or damaged, refer to the following procedures as necessary.

•	Brake Pedal Assembly Replacement

•	Power Brake Booster Replacement (for a bent or damaged pedal pushrod replacement)

Did you find and replace any worn, missing, misaligned, bent or damaged brake pedal system components?

Go to Step 11

Go to Step 12

Reinspect for proper brake pedal travel. Refer to Brake Pedal Travel Measurement and Inspection .

Is the brake pedal travel distance within the acceptable limits?

Go to Step 14

Go to Step 12

1. Inspect the power steering drive belt for conditions which could cause slipping.
2. Replace the power steering drive belt if necessary. Refer to Drive Belt Replacement in Engine Mechanical - 8.1L.
3. Check the power steering system fluid level and adjust the level if necessary. Refer to Checking and Adding Power Steering Fluid in Power Steering System.

Are the operations complete?

Go to Step 13

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1. With the ignition OFF, depress the brake pedal four times or until the effort required to apply the pedal increases significantly.
2. Maintain moderate foot pressure on the brake pedal and start the engine. Observe pedal operation.

Did the brake pedal fall slightly, then hold after the engine was started?

Go to Step 16

Go to Step 14

1. Inspect the power steering system pressure supplying the hydro-boost unit. Refer to Power Steering System Test in Power Steering System.
2. Replace power steering components as indicated by the results of the pressure analysis.

Did you find and correct a condition?

Go to Step 15

Go to Step 19

1. With the ignition OFF, depress the brake pedal four times or until the effort required to apply the pedal increases significantly.
2. Maintain moderate foot pressure on the brake pedal and start the engine. Observe pedal operation.

Did the brake pedal fall slightly, then hold after the engine was started?

Go to Step 16

Go to Step 19

1. Release the brake pedal.
2. Allow the engine to continue to idle in order to pressurize the system.
3. Turn the ignition OFF.
4. Wait 90 seconds, then depress the brake pedal two times to check for hydraulic brake booster immediate reserve. Observe pedal effort.

Did the brake pedal apply effort significantly increase when the brake pedal was applied the second time?

Go to Step 19

Go to Step 17

1. Release the brake pedal.
2. Start the engine to pressurize the system.
3. Turn the ignition OFF.
4. Wait 1 hour, then depress the brake pedal four times to check for hydraulic brake booster accumulator reserve leak-down. Observe pedal effort.

Did the brake pedal apply effort significantly increase when the brake pedal was applied the fourth time?

Go to Step 18

Go to Step 20

Replace the hydraulic brake booster accumulator assembly. Refer to Hydraulic Brake Booster Accumulator Replacement .

Did you complete the replacement?

Go to Step 20

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Replace the hydraulic brake booster assembly. Refer to Power Brake Booster Replacement .

Did you complete the replacement?

Go to Step 20

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Install or connect any components that were removed or disconnected during diagnosis.

Did you complete the operation?

Brake Assist System OK

Return to Symptom Table

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