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Home >> GMC >> 2021 >> Terrain Denali, FWD >> Repair and Diagnosis >> Brakes >> Mechanical - Hydraulic >> Hydraulic Brakes >> Diagnostic Information and Procedures >> Brake System Vacuum Source Test

Brake System Vacuum Source Test

    NOTE:
    • Temperature, accessory load, and elevation level will affect vacuum.
    • Vacuum readings will decrease by approximately 2.7 kPa (0.8 in Hg) for every 305 m (1000 ft) of elevation above sea level.
  1. Disconnect the vacuum hose from the vacuum brake booster check valve.
  2. Install a vacuum gauge to the vacuum hose.
  3. Start the engine and allow the engine to idle until normal operating temperatures are reached.
  4. With the vehicle in PARK, the engine idling, and the air conditioning (A/C) system OFF, check to see if the vacuum reading is within the specified normal vacuum range.

    Specification 

    47 - 68 kPa (14 - 20 in Hg)

  5. Turn the ignition OFF.
  6. If the vacuum reading was within the specified normal range, proceed to step 10.
  7. If the vacuum reading was NOT within the specified normal range, inspect the vacuum hoses for the following conditions:
    • Loose connection to the vacuum source.
    • Collapse, deformation or contamination
    • Cracks, cuts, or dry-rot
  8. If any of these conditions were found with the vacuum hoses, replace the hose, then repeat steps 2 - 4.
  9. If none of these conditions were found with the vacuum hoses, then there is an vacuum source problem, check the vacuum system.
  10. Remove the vacuum brake booster check valve from the booster.
  11. Install the check valve to the vacuum hose.
  12. Install the vacuum gauge to the check valve.
  13. Start the engine and allow the engine to idle in PARK with the A/C system OFF, until normal operating temperatures are reached.
  14. Turn the ignition OFF.
  15. Check to see if the vacuum reading is maintained within the specified normal vacuum range.

    Specification 

    47 - 68 kPa (14 - 20 in Hg)

  16. If the vacuum reading is maintained within the specified normal range, proceed to step 18.
  17. If the vacuum reading is NOT maintained within the specified normal range, replace the brake booster check valve, then repeat steps 11 - 15.
  18. Inspect the brake booster check valve grommet for the following conditions:
    • Loose connection to the vacuum brake booster
    • Deformation or contamination
    • Cracks, cuts, dry-rot
  19. If any of these conditions were found with the check valve grommet, replace the grommet.
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