Illinois CDL Practice Exam 2025 - Free CDL Practice Questions and Study Guide

The emergency air lines play a crucial role in maintaining the safety of the trailer's braking system. They are designed to provide air to the trailer's brake system in the event of a failure in the service lines. This allows the trailer's brakes to operate even if there is a problem with the primary braking system, thereby ensuring that the vehicle can come to a stop safely. Having a dedicated emergency air line means that there is a backup system in place, reducing the risk of accidents caused by brake failure.

The other functions listed do not accurately reflect the purpose of the emergency air lines. For instance, supplying power for lights or controlling stability are managed through different systems. Preventing brake overheating relates to the way brakes are designed and used, not the emergency air lines specifically. Therefore, the primary and correct function of the emergency air lines is indeed to provide reliable air supply in case of service line failure.