What happens when you push in the trailer air supply control knob? Question What happens when you push in the trailer air supply control knob? Answers Air is supplied to the emergency lines. The air system is disconnected from the trailer. The drain valve is opened. Air brakes are applied. Explanation To send air to the emergency (supply) lines, a driver can push in the trailer air supply control knob. The knob is red and eight-sided. This questions appears in the following tests: Georgia Double