At what air pressure range does the trailer air supply control (the red, octagonal, 'push to release' knob) pop out on its own, closing the tractor protection valve automatically?

AAMVA CDL Manual, Section 6, subsection 6.2.3 (p. 6-5), 'Trailer Air Supply Control'

This distinguishes the several pressure numbers covered in the air brakes unit — the 55 psi warning, the 20-45 psi spring-brake/pop-out range, and the 100-140 psi compressor cut-in/cut-out range — which are easy to mix up but represent very different stages of system health. Knowing exactly which number applies to which event is a frequent source of missed exam questions.