CDL Combination Vehicles TestCoupling and UncouplingMedium
When uncoupling a combination vehicle, after you have lowered the landing gear firmly to the ground and disengaged the fifth wheel locking mechanism, what is the next critical step before pulling the tractor forward?
- AConnect the glad hands to the dummy couplers on the tractor.
- BRelease the trailer's parking brakes.
- CDisconnect the air and electrical lines between the tractor and trailer.
- DPerform a light check on the trailer's brake lights.
Show answer & explanationAnswer & explanation
Correct answer: C. Disconnect the air and electrical lines between the tractor and trailer.
After the landing gear is supporting the trailer and the fifth wheel is unlocked, the air and electrical lines must be disconnected. Failing to do so before moving the tractor will damage the lines and potentially cause a loss of trailer brakes.
Why the other options are wrong
- A. Connecting glad hands to dummy couplers is done after disconnecting them from the trailer, protecting them from dirt and damage.
- B. Releasing trailer parking brakes is done during coupling to perform the pull test, or when preparing to drive, not during uncoupling at this stage.
- D. A light check is part of a pre-trip inspection, not a step in the uncoupling process at this stage.
Air/Electrical Disconnection
The act of detaching the glad hands (air lines) and the electrical cable that connect the tractor to the trailer, a crucial step in the uncoupling process.
- Prevents damage to lines.
- Must be done before pulling away.
- Ensures trailer brakes remain applied (if emergency line disconnected first).
Memory trick: Gear Down, Lock Off, Lines Out, Then Pull.