Air Brake Manual

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Pre-trip procedure for air single unit

Park the vehicle on level ground with the park brake set, the wheels blocked and the air tanks drained (if possible):

  1. Check security and condition of compressor, belts and air lines under hood.
  2. Start engine and let air pressure build up.
  3. With wheels blocked, release park brakes.
  4. Check brake adjustments (push rod travel) manually. Adjust if necessary.
  5. Verbally explain the proper procedure for adjusting an automatic slack adjuster.
  6. Governor operation (be sure spring brakes are released):
    • cut-out pressure between 120 and 145 psi (828 and 1,000 kPa)
    • cut-in pressure; fan brakes until compressor cuts in, usually about 20 to 25 psi (138 to 172 kPa) less than cut-out pressure, but at a minimum of 100 psi (690 kPa)
  7. At maximum pressure:
    • ensure the park brake is released
    • shut off engine
  8. Make and hold full foot-brake application:
    • maximum air loss after initial application must not exceed 1 psi in one minute
    • listen for audible air leaks
  9. With ignition key on, fan brakes to lower air pressure:
    • low warning system should operate at minimum 60 psi (414 kPa)
    • continue to fan brakes, truck park brake valve may shut off although on some vehicles the button may never close. Always ensure the spring brakes have been fully applied. On some vehicles the button may never close, however always ensure the spring brakes have been fully applied
  10. Run the engine between 600 and 800 rpm and observe the time needed for the air pressure to rise from 85 to 100 psi (586 to 690 kPa). It should be less than two minutes.
  11. Ensure park brakes are applied, remove wheel blocks.
  12. Two final tests:
    • apply park brakes and gently try to pull ahead; release park brakes
    • move slowly ahead and make foot-brake application


Rev: 2018