Step
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Action
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Yes
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No
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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.
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1
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Were you sent here from a Brake Symptom table?
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Go to Step 3
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Go to Step 2
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2
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Were you sent here directly from an electrical diagnostic to
investigate vacuum as a possible concern?
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Go to Step 5
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Go to
Symptoms - Hydraulic Brakes
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3
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Inspect for proper brake pedal travel.
Is the brake pedal travel distance within the acceptable
limits?
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Go to Step 5
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Go to Step 4
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4
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1. |
Inspect for worn, missing, misaligned, bent or
damaged brake pedal system components. |
| • |
Inspect the brake pedal bushings for excessive
wear and/or damage and inspect the brake pedal for a misaligned,
bent and/or damaged condition. |
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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?
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Go to Step 5
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Go to
Hydraulic Brake System Diagnosis to check for internal and
external fluid leaks and air in the hydraulic brake system
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5
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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?
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Go to Step 6
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Go to Engine Controls Diagnostics Information to check for vacuum
issues, including vacuum pump operation, if equipped
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6
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Does the condition occur during cold start-up conditions?
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Go to Step 7
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Go to Step 8
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7
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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?
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Go to Step 22
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Go to Step 9
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8
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During the vacuum source test, did the vacuum booster check valve
operate properly?
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Go to Step 10
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Go to Step 9
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9
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Replace the vacuum booster check valve. Refer to
Power Brake Booster Replacement .
Did you complete the replacement?
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Go to Step 10
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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?
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Go to Step 11
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Go to Step 12
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11
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Replace the vacuum sensor grommet.
Did you complete the replacement?
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Go to Step 12
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12
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1. |
If the vehicle is equipped with a vacuum sensor,
install a scan tool. |
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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?
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Go to Step 13
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Go to Step 16
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13
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Perform the vacuum booster leak-down test using a scan tool:
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1. |
Release the brake pedal. |
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2. |
Accelerate the engine to approximately 3000 RPM,
then release the throttle. Wait 2-3 seconds and repeat once. |
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3. |
Turn the ignition OFF. |
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4. |
Turn the ignition ON. Do not start the
engine. |
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Note: |
Vacuum readings will decrease by approximately 2.7
kPa (0.8 Hg) for every 305 m (1000 ft) of elevation above sea
level. |
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5. |
Observe and record the vacuum sensor reading on
the scan tool over a period of 60 seconds. |
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6. |
Determine the rate of vacuum leak. |
Was the rate of vacuum leak within the specified allowable
range?
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Go to Step 18
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Go to Step 14
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14
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1. |
Deplete the vacuum reserve by pressing the brake
pedal several times, or by removing the vacuum booster check
valve. |
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2. |
Separate the master cylinder from the vacuum brake
booster. Do not disconnect any brake pipes. Refer to .
Master Cylinder Replacement |
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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. |
Did you complete the operation?
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Go to Step 15
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15
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Perform the vacuum booster leak-down test using a scan tool:
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1. |
Release the brake pedal. |
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2. |
Accelerate the engine to approximately 3000 RPM,
then release the throttle. Wait 2-3 seconds and repeat once. |
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3. |
Turn the ignition OFF. |
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4. |
Turn the ignition ON. Do not start the
engine. |
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Note: |
Vacuum readings will decrease by approximately 2.7
kPa (0.8 Hg) for every 305 m (1000 ft) of elevation above sea
level. |
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5. |
Observe and record the vacuum sensor reading on
the scan tool over a period of 60 seconds. |
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6. |
Determine the rate of vacuum leak. |
Was the rate of vacuum leak within the specified allowable
range?
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Go to Step 22
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Go to Step 21
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16
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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 |
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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. |
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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?
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Go to Step 17
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Go to Step 18
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17
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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. |
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2. |
If equipped with a vacuum seal that requires
replacement and which is part of the booster, the booster assembly
must be replaced. |
Did you complete the replacement?
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Go to Step 18
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18
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Perform the vacuum boost functional test:
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2. |
If the engine was started after reinstalling the
master cylinder to the booster, perform the following: |
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3. |
Depress and maintain moderate foot pressure on the
brake pedal. |
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4. |
Start the engine and observe pedal operation. |
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5. |
Observe for audible vacuum leaks. |
Is there an audible vacuum leak coming from the brake pedal
area?
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Go to Step 21
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Go to Step 19
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19
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Did the brake pedal fall slightly after the engine was started,
then hold?
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Go to Step 20
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Go to Step 21
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20
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Perform the vacuum booster leak-down test:
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1. |
Release the brake pedal. |
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2. |
Accelerate the engine to approximately 3000 RPM,
then release the throttle. Wait 2-3 seconds and repeat once. |
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3. |
Turn the ignition OFF. |
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5. |
Depress the brake pedal 2 times to check for
vacuum booster available vacuum reserve. Observe pedal effort. |
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Both pedal applies should be assisted |
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The amount of assist could diminish slightly with
the second apply |
Were both pedal applies power assisted?
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Go to Step 22
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Go to Step 21
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21
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Replace the power brake booster. Refer to
Power Brake Booster Replacement .
Did you complete the replacement?
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Go to Step 22
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-
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22
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Install or connect any components that were removed or
disconnected during diagnosis.
Did you complete the operation?
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Brake Assist System OK
Return to Symptom Table
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