One new 3/4" female NPT x 3/4" male NPT in tank air compressor check valve. This style of check valve is usually screwed directly into the air tank, then the main line coming off the compressor is connected to it. This valve prevents backflow or tank pressure from getting back to the air compressor pump once the compressor is shut off. This valve has a 1/8" female NPT port on the side which usually goes to an unloader valve on your pressure switch or air compressor pump. If you do not have an unloader valve in your application, this port can just be plugged.