What is the purpose of the tractor protection valve on a combination vehicle, and when does it close automatically?

AAMVA CDL Manual, Section 6, subsection 6.2.2 (p. 6-5), 'Tractor Protection Valve'; 49 CFR 383.111(b)(3) (implications of severed or disconnected air lines between the power unit and the trailer(s)) and 383.111(b)(6)(iii) (tests that indicate the pressure

This valve is the reason a tractor doesn't lose all of its own braking air just because a trailer line fails; without it, a severed trailer line could drain the tractor's air supply too, leaving the driver with no brakes at all on any part of the rig. Understanding this fail-safe design is central to answering questions about what happens when air lines are cut or disconnected.