Home › CDL Practice Tests › Air Brakes Questions › Question 104
At about what pressure range will the tractor protection valve close and the trailer supply knob pop out during an air loss?
- A.At 120 psi during normal cut-out
- B.Roughly between 20 and 45 psi as pressure falls
- C.At 90 psi whenever the engine idles
- D.It never closes automatically
Correct answer
B. Roughly between 20 and 45 psi as pressure falls
Why
That range matches the spring brake application zone, coordinating trailer emergency braking with protection of the tractor supply. Seeing the red knob pop during the fanning test confirms the valve works.
Manual topic: Air Brakes - Spring Brakes and Parking Brakes · From the Air Brakes practice bank (AB-104).