Illinois CDL Practice Exam 2026 - Free CDL Practice Questions and Study Guide

The function of shut-off valves in a trailer's air lines system is primarily to isolate air lines when not towing another trailer. This is crucial because when a truck is not connected to a trailer, isolating the air lines helps prevent potential air leakage and maintains the integrity of the air brake system. By closing these valves, it ensures that the air pressure is contained within the system that is active, preventing unwanted loss and helping to maintain operational efficiency. This isolation is especially important for drivers who may need to connect and disconnect trailers often.