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

Measure the brake pedal travel. Refer to Brake Pedal Travel Measurement and Inspection.

Is the brake pedal travel distance within the specifications?

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 a 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 specifications?

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 specifications?

Go to Step 6

Go to Diagnostic Starting Point - Engine Controls

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. With the ignition switch in the OFF position, 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. Run the engine for 2 minutes.
3. Turn the ignition OFF.
4. Wait 15 seconds.
5. Pump the brake pedal 3 times in order to test the booster for available vacuum reserve. Observe pedal effort.

If operating properly, the vacuum brake booster should maintain assist for at least 2 pedal applications. Brake pedal travel will decrease on the second and the third brake pedal application.

Did the brake pedal travel decrease significantly on the second and third pedal applications?

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 procedures:

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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