CDL Combination Vehicles TestInspecting Combination VehiclesHard
During a post-trip inspection of a combination vehicle, you are checking the condition of the trailer's spring brakes. You notice that the parking brake knob on the dash is pushed in, but the trailer's spring brakes are still applied. What is the most likely reason for this discrepancy?
- AThe trailer's emergency air line is disconnected or has a severe leak.
- BThere is a leak in the service brake air line.
- CThe trailer's ABS system has detected a fault.
- DThe tractor's air compressor is malfunctioning.
Show answer & explanationAnswer & explanation
Correct answer: A. The trailer's emergency air line is disconnected or has a severe leak.
The trailer's spring brakes are applied when air pressure in the emergency (supply) line drops below a certain level (typically 20-45 psi). If the parking brake knob is pushed in (which sends air to release the spring brakes), but they remain applied, it indicates that air is not reaching or maintaining pressure in the trailer's emergency line, most likely due to a disconnection or severe leak in that line.
Why the other options are wrong
- B. A leak in the service brake air line would affect service brake application, not prevent the release of spring brakes when the parking brake knob is pushed in.
- C. An ABS fault would typically be indicated by a light and affect braking performance, not directly prevent spring brake release when air is supposed to be supplied.
- D. A malfunctioning compressor would affect overall air system build-up, but if the dash knob is pushed in and there's no air to the trailer, the emergency line is the more direct issue.
Trailer Spring Brake Release
Trailer spring brakes are released by air pressure supplied through the emergency (supply) air line from the tractor. A loss of this pressure causes them to apply.
- Parking brake knob (red) controls air to the emergency line.
- Pushing the knob in sends air to release spring brakes.
- Pulling the knob out dumps air, applying spring brakes.
Memory trick: Red knob in, air should flow; if not, emergency line's a no-go!