CDL Air Brakes TestAir Brake System PartsHard
A driver notices that the red low air pressure warning light and buzzer do not activate when the air pressure drops below 60 psi. However, the parking brake still automatically engages below 45 psi. What is the most likely issue?
- AA faulty air compressor governor.
- BA leak in the primary air tank.
- CA malfunctioning low air pressure warning switch or circuit.
- DIncorrect adjustment of the parking brake control valve.
Show answer & explanationAnswer & explanation
Correct answer: C. A malfunctioning low air pressure warning switch or circuit.
The low air pressure warning system (light and buzzer) should activate when system pressure drops below 60 psi (or usually 55 psi). If the parking brake still engages at its set point (typically 20-45 psi), it indicates the system is losing pressure, but the warning system itself is failing to alert the driver.
Why the other options are wrong
- A. A faulty governor would affect pressure build-up or cut-out, not the warning system's response to low pressure.
- B. A leak would cause the pressure drop, but the question focuses on the warning system's failure to activate, not the cause of the pressure drop itself.
- D. Parking brake control valve adjustment affects when the spring brakes engage, but the warning light/buzzer are separate safety features.
Low Air Pressure Warning System
A safety system (light and buzzer) that alerts the driver when air pressure in the service brake system drops below a safe operating level, typically 60 psi or 55 psi.
- Mandatory safety feature.
- Activates a red warning light and an audible buzzer.
- Different from the pressure at which spring brakes engage (20-45 psi).
Memory trick: Low pressure, loud warning- don't ignore the alarm!