Question The emergency air line supplies air to the: Answers Relay valves. Trailer air tanks. Signal line. Explanation The emergency air line (also known as the supply line) has two functions. The first is to supply air to the trailer air tanks, and the second is to control the emergency brakes on combination vehicles. Take a FREE Practice Test Now This questions appears in the following tests: Alaska Combination , Hawaii Combination , Louisiana Combination , Michigan Combination , Ohio Combination , Utah Combination ,