What should you do after connecting the air lines before backing under the trailer?

After connecting the air lines, the correct step is to supply air to the trailer system and then pull out the air supply knob. This procedure is crucial because it ensures that the trailer's air brakes are operational and that the system is properly charged with air before you attempt to back under the trailer. Supplying air to the trailer allows the brakes to release, which is a critical safety step before you can safely couple the tractor to the trailer.

By pulling out the air supply knob, you are also ensuring that the air is directed to the trailer's brake system, which prepares it for your connection. This action is essential for the proper functioning of the air brakes once the coupling is complete. It sets the stage for a safe connection and ensures that all systems are ready for operation once you are attached.

Other choices do not align with the correct procedural steps following air line connection. Inspecting the coupling mechanism, while important, typically happens after you are backed under the trailer to ensure the connection is secure. Disconnecting the air lines would be counterproductive at this stage since you want to establish a proper connection first. Checking tire pressure is also important but is not directly related to the process of coupling and should be done as part of a pre-trip inspection,