Final answer:
After connecting both air lines to a tractor and semi-trailer, you should check the air pressure in both the tractor and trailer's air tanks. Additionally, checking the air pressure in the tires of the tractor and trailer is also crucial.
Step-by-step explanation:
After connecting both air lines to a tractor and semi-trailer, you should check the air pressure in both the tractor and trailer's air tanks. This is important because the air pressure in the tanks ensures proper functioning of the brakes.
Checking the air pressure in the tires of the tractor and trailer is also crucial to make sure they are properly inflated. Proper tire pressure ensures improved traction, stability, and fuel efficiency.
By checking these air pressures, you can ensure the safety and efficiency of your vehicle before backing under the trailer.