Where are shut-off valves typically located?

Answer: B — On the service and supply air lines at the back of trailers

Shut-off valves are typically installed on the service and supply air lines at the back of trailers to control the flow of air to the braking system. This is crucial for maintaining the integrity of the air brake system, allowing drivers and maintenance personnel to isolate sections of the air system for repairs or emergency situations. When these valves are closed, they prevent air from reaching the brake components, which is essential for safety during maintenance. Understanding the locations of shut-off valves is important for drivers because it allows them to quickly respond to emergencies or perform maintenance without disrupting the entire system. While shut-off valves can also be found near fuel tanks for fuel line control, the most common and critical application related to CDL operations is found in the air system of trailers. The other locations mentioned in the options do not serve the purpose of shut-off valves in the context of trailer operations.