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What happens when you pull out the trailer air supply valve (put it in the emergency position) or its pressure is lost?
- A.The trailer brakes release so the trailer can roll freely
- B.Only the trailer's parking brake light turns off
- C.Air is cut off to the trailer and the trailer's spring brakes apply, which is also how the trailer brakes set when you park or disconnect
- D.The tractor's brakes apply instead of the trailer's
Correct answer
C. Air is cut off to the trailer and the trailer's spring brakes apply, which is also how the trailer brakes set when you park or disconnect
Why
The supply valve is the trailer brake's master switch: pressure in means spring brakes released and tanks charging, pressure out means spring brakes applied. That is why the red knob is pulled before uncoupling and pushed in only when lines are connected and you are ready to charge the trailer.
Manual topic: Combination Vehicles - Air Lines and Gladhands · From the Combination Vehicles practice bank (CV-243).