Final answer:
After coupling a tractor with a trailer, you must push in the air supply knob and release the brake before moving the tractor to ensure the trailer's brake system is filled with air and the parking brakes are released.
Step-by-step explanation:
When coupling a tractor with a trailer, it is crucial to follow the correct set of procedures to ensure safety and proper connection. After you have coupled the tractor with the trailer, you should not move the tractor until you have pushed in the air supply knob and released the brake. This step ensures that the air supply system fills the trailer's brake system and releases the parking brakes, allowing the vehicle to move safely without risking a disconnection or an unsafe situation due to the trailer's brakes being engaged while trying to move.