Where are shutoff valves commonly found?

Shutoff valves are commonly found in the service and supply air-lines of commercial vehicles. These valves play a crucial role in controlling the flow of air in the braking system, allowing the driver to manage air pressure effectively and ensure safe operation of the vehicle's brakes. By functioning within the air-lines, shutoff valves can isolate certain components of the air system for maintenance or repair, ensuring that the truck can be serviced without needing to deplete the entire system's air supply.

This is essential for preventing issues that could arise from a loss of air pressure and ensuring that the vehicle's braking systems remain reliable. The ability to control airflow effectively is critical for maintaining performance and safety standards in commercial driving.

The other options do not correctly identify the typical location of shutoff valves in a commercial vehicle. They may relate to various vehicle components, but they do not serve the purpose associated with regulating the air supply in the braking systems.