Which shut-off valves should be open when preparing to operate a trailer?

When preparing to operate a trailer, both the Emergency and Service Line Shut-off valves should be open to ensure proper operation of the braking system and the trailer's air supply. The Service Line Shut-off valve is responsible for controlling the supply of air to the brake system of the trailer, while the Emergency Line Shut-off valve is crucial for activating the trailer's emergency brakes in case of a disconnection or failure.

If both valves are open, the trailer is capable of receiving air pressure necessary for the brakes to function effectively and ensure safe handling during transit. This is essential for the driver to maintain control and adhere to safety regulations governing the operation of commercial vehicles.

Having only the Emergency Line or just the Service Line open can lead to the failure of critical brake systems, making it unsafe to operate the trailer. Additionally, if neither valve is open, the trailer cannot receive air pressure, rendering the braking system inoperable. Therefore, the proper configuration requires both valves to be open for safe operation.