An automatic inlet control mechanism and air compressor unit include a valve cavity and valve outlet. The valve cavity includes a valve control chamber and valve inlet chamber. A valve piston assembly is positioned between the valve control chamber and the valve inlet chamber to prevent the flow of air between therebetween. The valve outlet allows air to flow from the valve inlet chamber into the compressor unit. The valve piston assembly prevents air from flowing from the valve inlet chamber to the valve outlet when the compressor unit is not drawing air. A vent passageway allows air to flow between the valve control chamber and the compression cylinder inlet when compression is begun at the start-up of the compressor unit or at the loading of the idling compressor unit. A vent orifice restricts the flow of air from the valve control chamber to the compression cylinder inlet.