Question The service air line is attached to the: Answers Relay valves. Supply line. Emergency air line. Steering wheel. Explanation The service air line carries air and is controlled by either the foot brake or trailer hand brake. The service air line is attached to relay valves, which allow the trailer brakes to be applied quickly. Take a FREE Practice Test Now This questions appears in the following tests: Connecticut Double , Texas Double , Delaware Combination , Georgia Combination , Mississippi Combination , North Carolina Combination , Utah Combination ,