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 3

Go to Step 2

Were you sent here directly from an electrical diagnostic to investigate vacuum as a possible concern?

Go to Step 5

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 4

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.

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

Go to Step 5

Go to Hydraulic Brake System Diagnosis to check for internal and external fluid leaks and air in the hydraulic brake system

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 Engine Controls Diagnostics Information to check for vacuum issues, including vacuum pump operation, if equipped

Does the condition occur during cold start-up conditions?

Go to Step 7

Go to Step 8

Check engine vacuum, and vacuum pump operation, if equipped, under the same cold start-up conditions. Refer to Engine Controls Diagnostics.

Did you find and correct a condition?

Go to Step 22

Go to Step 9

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

Go to Step 10

Go to Step 9

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 10

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If equipped with a booster mounted separate vacuum sensor, inspect the vacuum sensor grommet for cracks, cuts, dry-rot, or damage.

Does the vacuum sensor grommet exhibit any of the conditions listed?

Go to Step 11

Go to Step 12

Replace the vacuum sensor grommet.

Did you complete the replacement?

Go to Step 12

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1. If the vehicle is equipped with a vacuum sensor, install a scan tool.
2. Using the Data Display function, determine if brake booster vacuum sensor data is available within either the chassis brake controls area or the powertrain engine controls area.

Is vacuum sensor data available through the scan tool?

Go to Step 13

Go to Step 16

Perform the vacuum booster leak-down test using a scan tool:

1. Release the brake pedal.
2. Accelerate the engine to approximately 3000 RPM, then release the throttle. Wait 2-3 seconds and repeat once.
3. Turn the ignition OFF.
4. Turn the ignition ON. Do not start the engine.
5. Observe and record the vacuum sensor reading on the scan tool over a period of 60 seconds.

Important: Vacuum readings will decrease by approximately 2.7 kPa (0.8 Hg) for every 305 m (1000 ft) of elevation above sea level.

6. Determine the rate of vacuum leak.

Specification
Maximum allowable vacuum leak rate: 10 kPa (3 in Hg) in 60 seconds

Was the rate of vacuum leak within the specified allowable range?

Go to Step 18

Go to Step 14

1. Deplete the vacuum reserve by pressing the brake pedal several times, or by removing the vacuum booster check valve.
2. Separate the master cylinder from the vacuum brake booster. Do not disconnect any brake pipes. Refer to Master Cylinder Replacement.
3. If equipped with a vacuum seal on the rear of the master cylinder, inspect the seal for damage and correct positioning on the master cylinder. Replace the vacuum seal as necessary.
4. Carefully reinstall the master cylinder to the brake booster. Refer to Master Cylinder Replacement.

Did you complete the operation?

Go to Step 15

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Perform the vacuum booster leak-down test using a scan tool:

1. Release the brake pedal.
2. Accelerate the engine to approximately 3000 RPM, then release the throttle. Wait 2-3 seconds and repeat once.
3. Turn the ignition OFF.
4. Turn the ignition ON. Do not start the engine.
5. Observe and record the vacuum sensor reading on the scan tool over a period of 60 seconds.

Important: Vacuum readings will decrease by approximately 2.7 kPa (0.8 Hg) for every 305 m (1000 ft) of elevation above sea level.

6. Determine the rate of vacuum leak.

Specification
Maximum allowable vacuum leak rate: 10 kPa (3 in Hg) in 60 seconds

Was the rate of vacuum leak within the specified allowable range?

Go to Step 22

Go to Step 21

1. Separate the master cylinder from the vacuum brake booster. Do not disconnect any brake pipes. Before removing the master cylinder, first deplete the vacuum by removing the check valve or applying the brake pedal several times. Refer to Master Cylinder Replacement.
2. If equipped with a vacuum seal on the rear of the master cylinder, inspect the seal for damage and correct positioning on the master cylinder.
3. If equipped with a vacuum seal that is part of the booster, inspect the seal for damage.

Does the vacuum seal exhibit any of the conditions listed?

Go to Step 17

Go to Step 18

1. If equipped with a vacuum seal on the rear of the master cylinder, replace the vacuum seal as necessary. Ensure the seal is positioned correctly during installation.
2. If equipped with a vacuum seal that requires replacement and which is part of the booster, the booster assembly must be replaced.
3. Carefully reinstall the master cylinder to the brake booster. Refer to Master Cylinder Replacement.

Did you complete the replacement?

Go to Step 18

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Perform the vacuum boost functional test:

1. If still separated, reinstall the master cylinder to the booster. Refer to Master Cylinder Replacement.
2. If the engine was started after reinstalling the master cylinder to the booster, perform the following:

With the engine OFF, depress the brake pedal several times to deplete the vacuum reserve.

3. Depress and maintain moderate foot pressure on the brake pedal.
4. Start the engine and observe pedal operation.
5. Observe for audible vacuum leaks.

The brake pedal should fall slightly, then hold.

Is there an audible vacuum leak coming from the brake pedal area?

Go to Step 21

Go to Step 19

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

Go to Step 20

Go to Step 21

Perform the vacuum booster leak-down test:

1. Release the brake pedal.
2. Accelerate the engine to approximately 3000 RPM, then release the throttle. Wait 2-3  seconds and repeat once.
3. Turn the ignition OFF.
4. Wait 60 seconds.
5. Depress the brake pedal 2 times to check for vacuum booster available vacuum reserve. Observe pedal effort.

•	Both pedal applies should be assisted

•	The amount of assist could diminish slightly with the second apply

Were both pedal applies power assisted?

Go to Step 22

Go to Step 21

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

Did you complete the replacement?

Go to Step 22

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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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