Home › CDL Practice Tests › Air Brakes Questions › Question 102
What does pushing in the red trailer air supply knob do?
- A.It sends air to charge the trailer tanks and releases the trailer spring brakes (when pressure is sufficient)
- B.It applies the trailer brakes harder
- C.It disconnects the gladhands automatically
- D.It drains the trailer tanks to zero
Correct answer
A. It sends air to charge the trailer tanks and releases the trailer spring brakes (when pressure is sufficient)
Why
The red knob is the trailer's air supply valve, and charging the trailer is what cocks back its spring brakes. If supply pressure later collapses, that same valve pops out and the trailer brakes apply.
Manual topic: Air Brakes - Spring Brakes and Parking Brakes · From the Air Brakes practice bank (AB-102).