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

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 5

Go to Step 3

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 Vacuum 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 4

Go to Step 5

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 8

Go to Step 5

Check the engine vacuum source that supplies vacuum to the vacuum brake booster. Refer to Brake System Vacuum Source Test .

Is the vacuum reading within the acceptable limits?

Go to Step 6

Go to Diagnostic Starting Point - Engine Mechanical in Engine Mechanical - 4.2L

During the vacuum source inspection, did the vacuum booster check valve operate properly?

Go to Step 8

Go to Step 7

Replace the vacuum booster check valve. Refer to Vacuum Brake Booster Check Valve and/or Hose Replacement .

Did you complete the replacement?

Go to Step 8

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1. Pump the brake pedal several times until the brake pedal becomes hard.
2. Maintain moderate foot pressure on the brake pedal and start the engine. Observe pedal operation.

Did the brake pedal drop slightly, then remain firm after the engine was started?

Go to Step 9

Go to Step 12

1. Release the brake pedal.
2. Turn the ignition OFF, then wait 15 seconds.
3. Pump the brake pedal two times to check for vacuum booster available vacuum reserve. Observe pedal effort.

(If operating properly, the vacuum brake booster should maintain assist for at least two pedal applications.)

Did the brake pedal effort increase significantly?

Go to Step 12

Go to Step 10

Inspect the brake pedal pushrod and the brake pedal for misalignment, a bent condition and/or damage.

Is the brake pedal pushrod and/or the brake pedal misaligned, bent or damaged?

Go to Step 11

Go to Step 13

Replace the brake pedal pushrod and/or the brake pedal, as necessary. Refer to the following procedure(s).

Did you complete the replacement?

Go to Step 13

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Replace the vacuum brake booster. Refer to Power Vacuum Brake Booster Replacement .

Did you complete the replacement?

Go to Step 13

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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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