Illinois CDL Practice Exam

The color blue is specifically used for the service line in a trailer's air line system. This standardization helps drivers and maintenance personnel quickly identify the function of the hoses, ensuring safety and efficiency during the coupling and uncoupling process. The blue color clearly indicates that it is meant for the service brakes, while other colors differentiate between different functions, such as emergency lines. This consistent use of color coding contributes significantly to the safe operation of commercial vehicles.