Question A shut-off valve: Answers Is located on the front of a trailer. Is located on the instrument panel. Prevents air from escaping an air brake system. Should always be open. Explanation 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. Take a FREE Practice Test Now This questions appears in the following tests: Colorado Combination , Indiana Combination , Massachusetts Combination , New Hampshire Combination , West Virginia Combination ,