D2 governor controls the pressure in the air system. The compressor runs continuously while the engine is running, so the D-2 governor activates the compressor's unloader mechanism when system pressure reaches the governor's cut-out pressure. It deactivates the unloaders once system pressure has dropped to the governor's cut-in pressure. While unloaded, the compressor simply pumps air from one cylinder to the other rather than send it to the tank.
Bendix brand (valves formerly manufactured by Westinghouse)