I am just about finished with building my own house. I have just installed a new Flotec pressure tank, G.E. filter, whirlpool water heater, all connected with pex piping. There is a shut off valve on the top of the filter which I shut for the purpose of filling the tank. In my 60ft well I have a 30 yr old 1/2 hp Franklin Electric well pump that was only used for 6 months, and has been sitting in the well for the last 30 yrs. I pulled the pump to make sure it works, and it does. I cleaned it thoroughly, put a new screen on it, put a new check valve on it, and everything worked fine. We tested the well and production of water. The well is not very strong, but does produce enough water to live with. I am now trying to fill the new flotec pressure tank and the tank will not fill. I ended up letting out all of the air in the tank and that tank filled. Then when I tried to put air back into the tank, the pressure built to about 25 lbs and then collapsed in about 5 seconds. I let out all the water again, refilled the tank with 28 lbs of air, and tried to pump the tank up. Nothing! I am frustrated right now because I have no idea what's going on. Any ideas?

You are going about it all wrong. You pressurize the tank (empty) to 2 lbs. below the cut on setting of the pump control switch, and then turn on the pump. If pump will not force water in the tank, the pump is either not strong to pump water under pressure or the well pipe has a leak.

If the new check valve is functioning properly and the pressure collapsed when you pressurized the tank after filling with water, you have a leak. Assuming all valves on the house side of the tank are closed and you do not see water anywhere, the leak would be on the well side.

Disconnect pipe on well side of tank, hold you hand over the end of pipe. Turn pump on. If you can stop flow of water with your hand either the pump is not strong enough or you have a leak (big one) in well pipe.

Sounds like a leak in well pipe