You have to measure the tank pressure when th epump turned off and a valve open until the water stops flowing. It should be charged to 2# less than the pump turn on pressure. The system pressure will go from the turn-on point to the max pressure then stop when the pump stops. If you dont' use any water, it should stay there. The tanks stores energy from the pump, and when you open up a valve, it drains off that stored energy when the water is discharged until the pump starts again. If it ever gets to zero in normal circumstances, something is wrong. If the tank, when check properly, gets to zero, there is a leak in the tank.