Before maintenance, which specific action must be taken regarding the trailer's air system?

Answer: C — Disconnect trailer air lines from vehicle

Before performing maintenance on a trailer's air system, it is essential to disconnect the trailer air lines from the vehicle to ensure safety and proper procedures. This action helps prevent unintentional air flow and pressure changes that could occur during maintenance tasks. By disconnecting the air lines, you eliminate the risk of unexpected system activation or release of air, which could lead to injury or damage. While it's important to drain air tanks, check for leaks, and test the pressure at other points in the maintenance process, these actions typically occur after ensuring that the system is isolated from any vehicle components. Disconnection allows for a more controlled repair environment and protects both the technician and the equipment.