MIL-V-23953C, MILITARY SPECIFICATION: VALVES, COMBINED VENT-CHECK, FOR SUBMARINE MBT BLOW LINES (SIZES 1-1/2 TO 2-1/2 INCHES) (11 MAY 1990)., This specification covers requirements for the construction, testing, and operation of combined vent-check valves with a nominal pressure seating of 4,500 pounds per square inch (lb/in 2), for use in submarine main ballast tank (MBT) high pressure air blow lines.