Home › CDL Practice Tests › Air Brakes Questions › Question 14
Which trailer air line carries the control signal that tells the trailer brakes how hard to apply?
- A.The service (control) line
- B.The supply (emergency) line only
- C.The electrical cord
- D.The drain line
Correct answer
A. The service (control) line
Why
The service line carries air in proportion to the driver's foot valve application and operates the trailer relay valve. The separate supply line keeps the trailer tanks charged and controls the emergency function if pressure is lost.
Manual topic: Air Brakes - Air Brake System Components · From the Air Brakes practice bank (AB-014).