In Tank Brass Standard Check Valve 1/2" Male NPT x 3/8" Female NPT3 8" Female x 1 2" Male NPT Brass Air Compressor In Tank Check Valve New One new 3 8" female NPT x 1 2" 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 pump is connected to it. This valve prevents back flow 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