What does the tractor protection valve do at the moment it closes because air pressure has dropped too low?

Explanation

The tractor protection valve exists to keep the towing unit's own air supply intact, whether that unit is a tractor or a straight truck, when a trailer breaks away or develops a bad air leak. When the valve closes, it stops air from leaving the tractor while releasing air from the trailer emergency line, which sets the trailer's emergency brakes. This is standard driver-training content on how the two systems interact, topic-scoped to 49 CFR 383.111(c)(3)(ii). Saying it shuts off the tractor's own service brakes backward-attributes the effect to the wrong system. The compressor-output option invents an automatic response not stated anywhere. The brake-lights option confuses this air-system protection feature with an unrelated electrical circuit.

Source: 49 CFR 383.111(c)(3)(ii)