CDL Combination Vehicles TestCoupling and UncouplingEasy
A driver is preparing to uncouple a combination vehicle. After ensuring the landing gear is fully extended and supporting the trailer, what is the next critical step before disconnecting the air lines?
- ALower the fifth wheel assembly.
- BDrive forward slightly to relieve pressure.
- CRelease the tractor parking brakes.
- DApply the trailer parking brakes.
Show answer & explanationAnswer & explanation
Correct answer: D. Apply the trailer parking brakes.
Before disconnecting air lines during uncoupling, the trailer’s parking brakes must be applied to prevent the trailer from rolling away once air pressure is released.
Why the other options are wrong
- A. Lowering the fifth wheel assembly happens later in the uncoupling process.
- B. Driving forward slightly is part of the final separation, not an action taken before disconnecting air lines.
- C. Releasing tractor parking brakes is incorrect; both tractor and trailer brakes should initially be engaged, then only trailer brakes remain on.
Trailer Brake Engagement (Uncoupling)
Before disconnecting air lines during uncoupling, the trailer's parking brakes must be applied to secure the trailer.
- Prevents trailer runaway.
- Ensures safety during air line disconnection.
- Critical step after landing gear is down.
Memory trick: Secure trailer first, then disconnect.