CDL Vehicle Combination Practice Test

After connecting the air lines, it is essential to supply air to the trailer system before backing under the trailer. This step is crucial because it allows the air brakes in the trailer to be charged with compressed air. By doing so, you ensure that the trailer's braking system is fully operational and ready to function properly once the combination vehicle is joined.

If air is not supplied prior to backing under, the trailer brakes may not engage or could be stuck, leading to serious safety risks during the coupling process and later when in motion. This key procedure helps mitigate issues related to braking performance, contributing to overall operational safety.

While checking the brakes, inspecting the trailer, and testing the lights are all important parts of pre-trip inspections and preparation, they should ideally be conducted prior to connecting the air lines or after air supply has been ensured. The focus here is on ensuring that the air system is charged and ready as a preliminary step before any further actions.