Final answer:
After shutting off the trailer air supply and locking the trailer brakes, the correct step in uncoupling a trailer is to lower the trailer landing gear. This ensures the trailer remains stable and secure upon disconnection.
Step-by-step explanation:
When uncoupling a trailer, after shutting off the trailer air supply and locking the trailer brakes, you should lower the trailer landing gear.
This is a crucial step in the process to ensure that the trailer is secure and stable when disconnected from the truck. After lowering the trailer landing gear, you can then disconnect the air lines and electrical connections, pull the release handle to disengage the fifth wheel lock, and gently pull the truck away from the trailer.