If you are trying to do a Retained Air Pumper, and need a reliable way to repeat the pressure inside the valve, to know when to stop pumping, a better way is to let the pump handle rebound after each pump, and mark the tube where some part of the linkage stops.... The higher the pressure inside the valve, the further the pump arm rebounds, and this is a pretty accurate way of getting the same pressure each time you partially refill.... You won't know what that pressure IS, but you will be able to be consistent....
hope that helps , i had one bookmarked and cannot find it.. it was years ago.. I generally make these type of gauge holders by hollowing threaded steel bolts.
the guy said its a tank to 1/8npt adapter, not what you need.