What are the functions of the emergency air line on a combination vehicle?
Correct answer: C – It supplies the trailer tanks and controls the trailer emergency brakes
The North Dakota CDL manual identifies two functions of the emergency or supply line. It fills the trailer's air tanks and controls the emergency brakes on combination vehicles. A loss of pressure in this line causes the trailer emergency brakes to apply.
Source: North Dakota Commercial Driver License Manual (2025–2027) — § 6.2.4, “Trailer Air-lines”

