Home › CDL Practice Tests › Air Brakes Questions › Question 118
When uncoupling a trailer, why is the trailer air supply knob pulled (or parking set) before disconnecting the gladhands?
- A.To apply the trailer spring brakes so the trailer stays put when its air lines are separated
- B.To charge the trailer tanks to maximum
- C.To turn off the trailer lights
- D.To release the fifth wheel lock
Correct answer
A. To apply the trailer spring brakes so the trailer stays put when its air lines are separated
Why
Setting the trailer brakes first guarantees the trailer cannot roll the moment the lines and then the tractor are removed. Skipping that order is how dropped trailers roll into docks, roads, and people.
Manual topic: Air Brakes - Spring Brakes and Parking Brakes · From the Air Brakes practice bank (AB-118).