It's not a sealed system. It needs to breath but instead of drilling a hole, you remove the 'o' ring from the brass end plug (on the S200 cylinder) before you refit it and don't tighten the end plug totally shut. You leave a small gap, about the thickness of a sheet of paper.
Hmm... that means having to empty the air tube each time I need to alter the firing valve spring.. Ah well. I'll start with the hammer spring and see how far I can back it off and what the outcome is before touching the valve gate spring again.