Air compressor check valve, 1/2" Compression x 1/2" MPT with 1/8" hole; plug hole if not needed.  https://mastertoolrepair.com/1-8-blk-steel-plug-s-h-100b6e/ If the small unloader (bleeder) valve on or under your pressure switch leaks constantly...

Old Part Numbers: 60-4-61-4, 058-00171/4" brass compression nut and sleeve. Order two, one for each end of the tube.

Air compressor brass compression adapter, 1/4" compression x 1/8" male NPT.Specifications 1/4" Compression x 1/8" MPT

Complete pump assembly.