CDL General Knowledge TestAir Brakes (if applicable)Easy
After stopping, a driver hears a continuous hissing sound from the general area of the air tanks. The air pressure gauge shows a slow but steady drop in pressure. What is the most likely cause?
- AThe air governor is stuck in the 'cut-in' position.
- BA leak in an air tank or air line.
- CThe low air pressure warning system is malfunctioning.
- DThe air dryer is purging excess moisture.
Show answer & explanationAnswer & explanation
Correct answer: B. A leak in an air tank or air line.
A continuous hissing sound combined with a steady drop in air pressure is a classic indication of an air leak, most commonly in an air tank, air line, or fitting.
Why the other options are wrong
- A. A stuck governor would prevent the compressor from cutting out, potentially leading to over-pressurization or continuous compressor operation, not a steady pressure drop with a hiss while stopped.
- C. A malfunctioning warning system would affect the warning, not the actual loss of air pressure itself.
- D. The air dryer purges with a distinct, short 'whoosh' or 'hiss' sound, but it's intermittent, not continuous, and shouldn't cause a significant, steady pressure drop when the system is otherwise sealed.
Air Leak Detection
The process of identifying sources of air loss in an air brake system, often indicated by audible hissing or a pressure drop.
- Leaks reduce braking effectiveness.
- Can be found using soapy water or listening.
- Requires immediate repair to ensure safety.
Memory trick: Hiss + Drop = Leak's Got a Job!