I have been in my house for about a year, and I am having an intermittent problem that I hope somebody on here can help me with.

I have an above ground pump, with about a 20 gallon pressure tank (the kind with the automotive valve stem on top). The pump has a pressure gage but the tank does not. The pump cuts in at about 50 psi and cuts out at about 70 psi. The tank does have air pressure in it and I do not get any water out of the valve stem. Downstream of the pressure tank is a water conditioning system and then a water softener. Both of those systems have bypass valves, and they both share a common overflow/drain that is plumbed through the wall and empties into a downspout on the exterior of the home. That is where water goes when those systems do a periodic regen cycle, etc.

Several times last winter I noticed the pump running for extended periods of time. Like tens of minutes. A couple of times maybe an hour. When I have checked the system at that time, I notice that the pressure tank is empty (just by feel when I rock it slightly) and the gage on the pump is reading very low, which of course is why the pump stays on. But even though the pump is on, it is not filling the pressure tank. I eventually noticed that water was draining out through the drain/overflow for the conditioning systems instead. I didn't know why, but each time it would eventually correct itself and about the time I really started paying attention, this stopped happening for quite awhile. I had tinkered with the cut-in/cut-out settings for the pump but I don't think that had anything to do with fixing it. For the first time in months this happened again this morning. While I was taking a shower I noticed the pressure got low really quick. I got out of the shower, the pump was running and the pressure tank was mostly empty. The gage on the pump was reading maybe 20 psi. I noticed that water was going out through the drain/overflow. As I stood there for a minute, I realized the pressure gage on the pump was still dropping and sure enough, the pressure tank was actually emptying. No water on anywhere in the house. The water from the pressure tank and the pump were emptying through the conditioning systems overflow/drain to outside. What a waste! I opened the bypass valves on both of the conditioning systems, which I hoped would close off the drain to the outside, and I think it did because about that time the pressure tank started filling and the pressure gage on the pump started going up and before long reached the cut-out pressure and shut off. I then un-bypassed the conditioning systems and everything was fine, no water from the pressure tank went out the overflow.

Needless to say I am tired of wasting $$ on water and electricity for this issue (and I don't want to burn up a well pump because it is running for so long) and I would like to get this resolved! Can anybody point me in the right direction for why this would happen? It looks to me like the pump itslef is working exactly as it should, but the pressure tank isn't filling which is forcing water out the overflow instead. Is there a valve of some sort on the inlet/outlet of the pressure tank that could be sticking? Any assistance would be greatly appreciated.
Dave