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NM CDL Air Brakes study materials
NM CDL Air Brakes study resources

NM CDL Air Brakes: Air Brake System Malfunctions and Emergency Procedures

NM CDL Air Brakes · Work through your exam, one topic at a time.

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Air Brake System Malfunctions and Emergency Procedures

This domain tests recognition of air loss, low-pressure warning, spring-brake application, brake fade, trailer breakaway, ABS faults, and emergency stopping choices. The exam focuses on early, safe action.

Core rules

  • The low-pressure warning must activate before pressure falls below 60 psi. When it does, stop safely and find the leak. Do not continue to the next stop.
  • Spring brakes apply automatically at about 20–45 psi. Application may be gradual, and rear wheels can lock. Do not wait for the spring brakes to stop the vehicle in traffic.
  • If air pressure is lost, downshift, use engine braking or a retarder if available, and look for an escape ramp or safe pull-off. Use the parking brake gradually at low speed, not suddenly at highway speed.
  • Brake fade from overheating causes a hard pedal, smell, or smoke. Stop, cool the brakes, and use a lower gear. Do not continue downhill at speed.
  • A trailer breakaway or supply-line failure should be protected by the tractor protection valve. Know that the emergency line and trailer spring brakes are part of that protection.
  • An ABS malfunction lamp means ABS is disabled; normal service brakes remain. Do not pump the brakes.

Worked example

A driver sees the gauge at 80 psi and falling, with the low-pressure warning expected at 60 psi and an exit one mile ahead. Reasoning: the driver should slow and leave the road before 60 psi, not try to reach the shop. If pressure reaches 20–45 psi, spring brakes may apply suddenly and lock wheels; stopping early avoids that hazard.

Traps that cost marks

Do not treat the warning as a reminder to stop later. Do not cage spring brakes to drive with a leak. Do not use the trailer hand valve as an emergency brake. Do not pump ABS. Do not ignore a gauge that reads zero or fails to build.

Key points

  • Low-pressure warning by 60 psi: stop safely.
  • Spring brakes apply at about 20–45 psi.
  • Air loss: downshift, use engine braking, seek an escape ramp.
  • Brake fade: stop, cool, use a lower gear.
  • ABS lamp on: normal brakes remain; do not pump.