Sounds like you are talking about galvanized pipe. If that is the case, others on this board could address that better than me. But I will say this much. If it was me, and the leak was a few drops an hour and not in a position to damage anything, then I wouldn't worry about it. My experience has been that galvanized pipe will seal small leaks by rusting over time. I might get shot down for saying this, but I have seen this happen in several places. And the only alternative is to start at the union and work your way back to the offending joint. Of course, doing that is not the end of the world either. BTW, did you use a brass "T" fitting under the new tank to hook everything up? Also, what size tank are you dealing with?
I would suggest that you address your pressure issues. Having the pump not want to cut off is not something you want to play with. Remember with wells that the stuff above ground is a heck of a lot easier to deal with than the stuff below the ground. If you keep running that pump when it will not cut off because the system will not get to cut off pressure, you will shorten the pump's life considerably and could end with below ground issues as opposed to what you have now. The fact that the pump continues to run but will not get above 38# is troubling. The first step is to set the pressure correctly in the tank. Again, that should be 2# below the cut in pressure and should be set when the tank is basically empty.
Check out this post for more info on the pipe leak. Speedball generally gets it right.
https://www.askmehelpdesk.com/plumbi...ak-180538.html
One more thought. I did exactly what you are doing a few years ago. The old galvanized tanks with no diaphragm had to have some method of keeping air in the tank. Many of them used a system which allowed water to drain back down the well pipe every time the pump shut off (submersible pumps only). When the pump turned back on, it would first deliver all the air in the well pipe into the tank, thus recharging the air in the tank. A valve on the side of the tank would keep the tank from becoming completely full of air. If this was not done, the air in the tank would, in a few months, dissolve into the water and the air charge in the tank would be gone... bad news. Diaphragm tanks don't need this since the diaphragm keeps that air and water separated. When you replace a galvanized tank with a diaphragm tank, you must be sure that this system, if its in place, is disabled.
Your diaphragm tank probably fills/drains from one port in the bottom, thus doing away with the problem of water entering/exiting from 2" up. No crud on the bottom of the tank! You are going to like this new tank!