What will occur if the driver crosses the air lines when hooking up to an old trailer?

Crossing the air lines when hooking up to an old trailer leads to a situation where the brake system will not function properly. Air brakes rely on a specific configuration of lines to deliver compressed air effectively to the brake chambers. If the lines are crossed, the air will be misdirected, causing the brakes to fail to engage as intended. This can create a dangerous situation since the driver will not be able to stop the trailer effectively.

The other options do not directly relate to the consequence of crossing air lines. For example, while it is true that trailer lights could potentially fail to function due to issues unrelated to the air lines, this is not the primary consequence of crossing them. Similarly, engine overheating is unrelated, as is the perfect functioning of the brakes, which is not possible under these circumstances. Understanding the proper connection and functionality of air systems is crucial for safe trailer operation.