The emergency air line controls the emergency brakes on combination vehicles, and _________.

Emergency air line: The emergency line (also called the supply line) has two purposes: (1) To supply air to the trailer air tanks, and (2) to control the emergency brakes on combination vehicles. Loss of air pressure in the emergency line causes the trailer emergency brakes to come on. The pressure loss could be caused by a trailer breaking loose, thus tearing apart the emergency air hose. Or it could be caused by a hose, metal tubing or other part that breaks, letting the air out. When the emergency line loses pressure, it also causes the tractor protection valve to close (the air supply knob will pop out).