Most Pressure Tanks made today have a rubber bladder that is filled with water from the Well Pump. It can eventually rupture over time or from improper air pressure settings. When the Bladder ruptures, water from the well will start to steal a little bit of air from the tank with every pump cycle eventually filling the tank full of water. If you push the tank from the top slightly sideways to gauge the weight of the tank, you should be able to see if it feels full or nearly empty (which is how it should feel if it’s still working properly).
Another way to check your tank is to push the little stem within the air valve which is usually on the top or near the top of the tank. It will look just like the valve on your car’s tire. By pushing the stem in, you should be letting a little air out. If water comes out instead of air, your tank is definitely bad.
If neither of the above methods work for you, turn off your pump and open a faucet somewhere to let all the water pressure out of your plumbing system. Take a tire gauge and check the air pressure in the tank. It should be two pounds less than the turn on pressure of your pump. If it is not at this pressure but you do have some pressure left, chances are the bladder is still good and you can add the proper amount of air and continue using the tank. If you have no pressure left, there is a good chance the bladder has failed, but not necessarily. Try adding air into the tank to the correct pressure you are looking for. At this time any water left in the tank should blow out, if not tank is bad. If the water does blow out and air blows out after it, tank is bad.