A shut-off valve:

Question

Shut-off valves are used in air supply lines to control the passage of air from one trailer to another. The rear shut-off valve should always be closed to prevent air from leaving the braking system.

Accepted Answer

Prevents air from escaping an air brake system.

Answer

Is located on the front of a trailer.

Answer

Is located on the instrument panel.

Answer

Should always be open.