Well problems-no pressure

22 months ago I had the old pump in my well replaced with a new 3/4 hp Red Jacket submersible pump. I don't know how deep the well is. About a year ago, I had the rusted out metal tank replaced with a new fiberglass tank which is underground. Recently, I started having problems. The gauge shows no pressure in the tank. I could hook my small air compressor to the valve and run it to get a good flow of water in the house, but it only lasted long enough for my wife and I to shower before it started spitting air. I dug down to the tank, found a few slow leaks in the connections from the top of the tank to the pressure switch. I took every thing apart and reconnected it with new teflon tape. It still did the same thing. I replaced the copper line from the tank and all fittings up to the pressure switch which I replaced with a ProPlumber 30-50 psi 2 wire switch. I purged the tank by hooking my compressor to the valve and opening the faucet outside that has a water hose attached. With the power to the switch (and Pump) off, I ran the compresor until no water came out of the hose, only spitting air. I then shut the faucet off and added approx 28 lbs of air to the tank. I then turned the power back on. However, the tank would not hold air. It would get up to the correct pressure so long as I had the compressor running, but without it the pressure would drop to close to nothing...maybe 2 or 2 psi. I have checked every connection that I made repeatedly with soapy water and found no leaks. What could be the problem??