I would not necessarily trust the shut off valve, as they can erode and allow a small amount of water through. You could check at closest valve. If the tank is in the house, that could also be the issue. There should be an air valve. Check it to see that there is no water coming out when Schrader valve is depressed. If you have water the tank is shot (diaphram has ripped making a waterlogged tank) the air pressure in the tank at the Scrader valve should be 2# less than the cut in of the pressure switch. As you let air out to check for water, you would have to pump back upThis will cause cycling as well. You could have had an air leak, little gremlins listening to the cool hiss, or a bad air valve> I guess I should have started with if its a diaphram type tank which the greatest bulk have been for the last 20 years.
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