The air will compress until the water pressure and air pressure in the tank are equal. If you don't precharge the tank, you will end up overstretching the bladder and causing it to likely fail early, plus, if there is no air pressure in the tank ,there is nothing to push the water out of the tank and your pump will need to run with any valve opening. Since water doesn't compress for all practical purposes, to get the pressure to stop the pump, you are expanding pipes, hoses, etc a very small amount. Since that probably only takes a few teaspoons of volume, when you open a valve, it drops instantly, and the pump comes on.
Schrader valves aren't designed for wet environments...
If you have one of those valve covers that has a split head sticking out, it is used to tighten or loosen the valve from the shell. Yours could be loose, and would leak air. After filling it up, when you draw water into a glass, do you see a lot of small bubbles in it? Or, did you get any spurts of air when you opened a faucet?





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