Air compressor check valve. 3/8" Comp x 1/2" MPT with 1/8" FPT side port.  The check valve is a one-way valve that prevents air from back-feeding from the air tank. If your unit has a hard time restarting when trying to cycle or your pressure switch...

Order this if ordering valve plate or a new head.

Old Part Numbers: Z-D26611If your valve is leaking on the side of the switch, do not order this part. Order check valve instead.

Old Part Numbers: D24888, D30198, 5135135-00