If the tnak does not have a tire pump air valve, then turn off the pump, drain the tank, (remove one of the plugs in the tank to allow complete drainage, and the relace the plug before turning the pump back on. This is the way tanks were initialized before there were bladder tanks.: Just bought new house.Need advice on how to fill pressure tank (appears to be older, non bladder type)with air.What is the purpose of filling it?Should I drain the tank of water before I fill it?How do I know when it is 1/2 full?What happens if I overfill it?Can you please explain the basics and give me some advice on maintaining it.Thanks.