The Bendix® D-2 governor operates in conjunction with the air compressor unloader mechanism. The governor automatically controls the air pressure in the air brake supply system between a maximum cut-out pressure and a minimum cut-in pressure. The air compressor runs continually while the engine runs. The governor actuates the air compressor unloader mechanism, controlling the actual compression of air. The mechanism stops or starts the compression of air when the maximum or the minimum air reservoir pressures are reached.