CDL General Knowledge TestCombination Vehicles (if applicable)Hard
When performing a pre-trip inspection, a driver notices that the spring brakes on the trailer are not applying when the emergency air line (red glad hand) is disconnected. What does this indicate?
- AThe trailer's emergency braking system is faulty.
- BThe trailer's service brakes are working incorrectly.
- CThe parking brake control in the cab is activated.
- DThe trailer's air tanks are over-pressurized.
Show answer & explanationAnswer & explanation
Correct answer: A. The trailer's emergency braking system is faulty.
Disconnecting the emergency air line should cause the trailer's spring brakes to apply automatically, bringing the trailer to a stop or holding it in place. If they do not apply, it signifies a critical failure in the trailer's emergency braking system, rendering the vehicle unsafe to operate.
Why the other options are wrong
- B. This test specifically checks the emergency/parking brakes, not the service brakes directly.
- C. While the parking brake control operates the emergency brakes, the test is to ensure the automatic application when air pressure is lost (simulated by disconnecting the red line).
- D. Over-pressurization would be relieved by the safety valve, and would not prevent spring brake application upon air loss.
Trailer Emergency Brake Test
Disconnecting the emergency air line should apply the trailer's spring brakes. Failure to do so indicates a critical fault in the emergency braking system.
- Red glad hand = emergency.
- Disconnecting applies spring brakes.
- Failure = unsafe condition.
Memory trick: Red line off, brakes don't bite, something's wrong, not right.