RE: Valve Screws - Suggestion for a better design
very interesting points.
Actually I am not much worried about the O-ring being used in the wrong way as it actually acts, as you delineated, as a safety valve, letting some pressure go when the board is heated and letting just a little water in actually when it is cooled/depressurized.
What I really don't like about the O-ring system (even if as a graduated engineer I don't like much such incorrect solutions) is the fact that if, or better say: "when", some sand gets in the hole, the O-ring will not even came close to seal anything, plus it is not easy to detect nor to clean it.
Maybe a good solution would be to use tapered self-sealing screws. My first sandwich board, a Phil McGain proto, just had a small self tapping screw in a piece of plastic, no gaskets, and it worked flawlessly.