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At about what pressure will the trailer air supply control pop out (or the tractor protection valve close) as system pressure falls?
- A.Commonly in the range of about 20 to 45 psi as pressure is lost
- B.At normal operating pressure around 120 psi
- C.Only at exactly zero psi in every design
- D.At 90 psi, when the compressor cuts in
Correct answer
A. Commonly in the range of about 20 to 45 psi as pressure is lost
Why
The control is set to close while the tractor still holds a reserve of air, not after every tank is empty. The commonly taught range for that automatic pop-out is roughly 20 to 45 psi, varying by system design.
Manual topic: Air Brakes - Tractor Protection Valve · From the Air Brakes practice bank (AB-233).