During my AC397 buiild , I had every thing go wring, except having it exit out the pump tube under pressure.
you seem to be convinced that the or is holding, so lets rule that aout. Do you cock it before starting to pump? Make sure the check valve isnt "sticking" open after a very few stroke. And add a wrap or two of teflon plumbers tape to the screw/bolt that holds the grip frame and valve together. If that works, then either the bolt threads have been worn or the hole in the valve is worn and getting big enough to allow some air to pass, especially under pressure.
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