CDL Air Brakes TestAir Brake System PartsEasy
A driver is performing a brake test and notices that the red low air pressure warning light and buzzer activate when the air pressure drops to 70 psi. What does this indicate?
- AThe brake pedal (foot) valve is stuck open.
- BThe low air pressure warning system is functioning correctly.
- CThe vehicle is equipped with an older air brake system.
- DThe air compressor is not building pressure fast enough.
Show answer & explanationAnswer & explanation
Correct answer: B. The low air pressure warning system is functioning correctly.
The low air pressure warning system is designed to alert the driver when the air pressure in the system drops below a safe operating level, typically between 55 psi and 75 psi. An activation at 70 psi falls within this acceptable range, indicating correct operation of the warning system.
Why the other options are wrong
- A. A stuck-open foot valve would cause a rapid loss of air, but the activation of the warning system itself at 70 psi indicates the warning system is working, regardless of the cause of pressure drop.
- C. The warning threshold is standard across many air brake systems, not specific to older ones.
- D. This symptom does not directly indicate compressor speed, only that a low pressure threshold was met.
Low Air Pressure Warning
The low air pressure warning system alerts the driver when air pressure in the brake system drops below a safe operating level (55-75 psi), typically with a red light and buzzer.
- Mandatory safety feature.
- Activates between 55-75 psi.
- Indicates insufficient air for safe braking.
Memory trick: Low pressure: 70 is fine, below 55 is danger time.