DC-4 double check valve. P/N 278599. NSN 2530009818736.
Female threaded ports are 3/8 inch pipe, male threaded port is 1/2 inch pipe.
Used on various versions of the M939 and LET/MET series.
Double check valves are used in an air brake system to direct a flow of air into a common line from either of two sources, whichever is at the higher pressure. They may be used for directing air flow for specific functions or to select the higher pressure of either of two sources of air as a supply source.