Question What does the trailer air supply control look like? Answers It is a square blue knob. It is a triangular yellow knob. It is an eight-sided red knob. It is a circular orange knob. Explanation On newer vehicles, the trailer air supply control is an eight-sided red knob. It's used to control the tractor protection valve. Take a FREE Practice Test Now This questions appears in the following tests: Texas Double , Minnesota Air Brakes , Montana Combination , New Jersey Combination , Pennsylvania Combination , Utah Combination , Wyoming Combination ,