Final answer:
Crossing the air lines when hooking up a trailer to a tractor can lead to a malfunctioning brake system, potentially causing unsafe driving conditions and increasing the risk of accidents or vehicle damage.
Step-by-step explanation:
If a driver crosses the air lines when hooking up to an old trailer, this means that the trailer's brake system may not function as intended. The air lines on a tractor-trailer are crucial for controlling the trailer's braking system. There are typically two lines: the service line, which is controlled by the foot brake and sends air to apply the trailer brakes, and the emergency line, which supplies air to the trailer's air tank and activates the brakes in case of a loss of pressure.
When these lines are crossed, the system's operation will be reversed or compromised. For example, applying the brake pedal could send air to the wrong line, failing to activate the trailer brakes properly or even causing unexpected brake behavior.
This can result in dangerous driving conditions, leading to an inability to stop the vehicle effectively, which may cause accidents or damage to the vehicle. Therefore, it is critical to ensure that the air lines are correctly connected when hooking up a trailer.