Final answer:
Before backing under a trailer, it is essential to check for air leaks and secure connections in the air line system to ensure road safety and prevent accidents.
Step-by-step explanation:
After you connect the air lines, but before you back under the trailer, you should ensure there are no air leaks and that all connections are secure.
This is a crucial safety step in the process of connecting a tractor to a trailer in the context of commercial driving or truck operation. It is important to listen for air leaking and to check the air pressure gauges to confirm that the system is properly pressurized before proceeding to back under the trailer. Doing so helps prevent potential trailer separation or brake failure, which can lead to serious accidents on the road.