This is for tanks that have a bladder inside which holds the water only. The air in the tank is above the bladder and applies pressure to the bladder using the wall of the medal tank as the envelope for the air. Most come pre-charged from the factory. Air and water NEVER mix in there. The valve stem is the entry point for the air is is NOT ATTACHED TO THE BLADDER. I had an air leak somewhere in my tank system. Worst case the bladder was ruptured but I had to check. I had the pump recycle every 3 seconds. I must not have had any pressure in the tank. Checked it and I was right. Re- pressured it at the valve stem on top according to manuals instructions and then waited for a day to see if it held pressure 24 hours. It did not hold, so I got some liquid dish soap and spread some around the valve and it bubbled a lot. NOTE: It could leak from the stem itself which is replaceable ( the un-screw-able cap covering the metal stem inside the rubber tube/stem.) You'll need a tire stem removal tool from Walmart for 50 cents. Usually in a packet with both the tool and a new stem. Looks like a cap with a forked prong on the back side. Mine leaked around the base of the stem where it enters the tank. I went on line to see if there is a remedy and most say no and replace entire tank. These things are hundreds of dollars...all for a $2 valve. So I let all the pressure out of the tank by uncapping the stem tower and released the pressure by pushing down inside on the plunger pin until all pressure is gone...or else the stem will shoot out like a bullet. Then I spread plenty of dish liquid around the hole entry area and gently tilted the stem all around in a circle. Kinda wiggling it in place to loosen up the rust grip. (Don't worry it won't come out. You would have to cut it to get it out. I suppose you probably could crack it if you rotate too extremely.) Use you own judgement here. All you are doing is getting the lube (soap) in the groove around the ring so you can get the rust grip loosened up so you can spin/turn the valve in place to loosen up the rust and gunk keeping it from sealing properly. I took a pair of vice grips and grabbed hold of the area just above the bulge where it comes out of the tank and turned it in place 2-3 rounds. Just enough vice grip to hold tight during the spin and not crush the tower. After the lube works in, it should seem easier to turn. Remember this is after you are ready to replace the tank and last ditch effort. Re-pressure the tank and check for leaks. You don't have to put water back in to test the air pressure. Remember it should be there even without water as from the factory. Air it up and apply soap to test. Mine has been OK for months now. A new bladder is almost as expensive as a new tank so that was a no go. I have located a replacement valve (see below). If you have been around car tubeless tires, this is the same thing on these tanks. They are inserted with a tool and are held in with rubber expanded in the hole rim after insertion. To get these out you have to cut them...that's how much the tool squeezes the rubber and seals the rim. They also can only go in from the under side...that's why these are so hard to do. Anyway, if you want to try even more last effort, you can take the tank out in the yard, and flip it upside down and remove the bladder. Several nuts hold the ring in place which clamps the bladder opening to the water entry point. Take the bladder out and look inside the tank. At the top end is where the valve is. Flip it over and cut the stem out of the hole. Then go to an auto parts store and get a replacement valve which can be inserted from the inside and tightened with a nut on top. Replace the bladder and air it up properly VERY IMPORTANT. I have found this one on Amazon which is a push it through the inside of the hole and do all the tightening from the top side. $8 for 4.

VTR TR-416 1 1/2" Outer Mount Metal Valve Stem (4 Pack)