My old gage has some water on the inside of the dial,and I need to change it. There's no shut off valve on the line coming in from the water pump to the tank so ill have to turn the pump off and drain the tank to replace the gage. I believe my tank has an air bladder because there's a valve on top of the tank to receive a hand held air pump. How do I adjust the air pressure in the bladder after my repair to what it is presently?