Home › CDL Practice Tests › Combination Vehicles Questions › Question 238
What does the service air line do?
- A.It carries air pressure controlled by the brake pedal (and the trailer hand valve) to apply and release the trailer service brakes
- B.It keeps the trailer tanks charged while parked
- C.It operates the landing gear
- D.It supplies the trailer ABS lamp with power
Correct answer
A. It carries air pressure controlled by the brake pedal (and the trailer hand valve) to apply and release the trailer service brakes
Why
Every normal stop sends its signal through the service line, which is why a kinked, crossed, or leaking service line shows up as weak, delayed, or absent trailer braking. The supply line keeps the trailer ready; the service line tells it when to brake.
Manual topic: Combination Vehicles - Air Lines and Gladhands · From the Combination Vehicles practice bank (CV-238).