What happens when you pull out the trailer air supply control (red knob)?
a.The trailer brakes release
b.Air supply is shut off to the trailer and the trailer emergency brakes apply
c.The engine shuts off
d.The tractor parking brakes release
Explanation
Pulling out the red trailer air supply control knob shuts off the air supply to the trailer and activates the trailer emergency brakes via spring force. This is used when parking or in an emergency to stop the trailer.