Illinois CDL Practice Exam.

Closing the shut-off valves in the air lines at the back of a trailer is a standard procedure when unhooking the trailer. This action prevents air from being supplied to the trailer's brakes, ensuring that they do not accidentally engage while being unhitched. This is a critical safety measure, as it helps to avoid potential accidents or injuries that could occur if a sudden engagement of the trailer brakes happened when the vehicle is not properly secured. In contrast to this, situations such as overloading the trailer, driving downhill, or accelerating quickly do not necessitate the closing of these valves. Overloading can indeed affect braking and stability, but it does not warrant disabling the air lines. Similarly, the other situations involve factors where the functionality of the brakes remains crucial, so closing the valves is not appropriate or necessary.