You apparantly are missing two parts: a check valve to keep the water in the tank, and a pressure switch to turn the pump off once the desired pressure is met. The pressure switch is typically adjustable, and will have a high/low differential of around 20 pounds, or 1.4 bar. You need to have the air pressure in the dry (no water, and a valve open) set about 2 pounds less than the turn-on pressure for the pump. You can usually buy pressure switches preset to 20/40, 30/50. or 40/60 pounds, but they are probably all the same switch, and can be adjusted. So, the pump would pump up to the max pressure and turn off. As you use the stored water under pressure, when it drops to the lower value, the pump turns back on. You want the size of the tank to give you enough time inbetween cycles for the pump to cool off and not cycle all of the time. You may find, if you are say filling a tub that the pump can't bring the pressure up to the max and will run during the time you have to water running.