If you get your water from a well, you need a pump to transfer water from the well to a holding tank, and unless you have a gravity-feed system, you need another pump to pressurize the water for use in the house. Systems such as this need a separate storage tank for pressurized water to prevent the pressure pump from cycling too frequently and wearing out. Tank and pump problems are related.
Bladder-style pressure tanks contain a rubber bladder that separates the water in the tank from a compartment of air. When the pump fills the tank with water, the bladder expands and compresses the air. The pump continues until the pressure in the tank reaches the desired value. For this system to work properly, the tank must be pre-charged by filling it with air at a pressure just below the cut-in pressure of the pump, which is the pressure at which the pump starts pumping. The tank has an air valve on the top for this purpose.
Tank Needs Charging
Just like an automobile or bike tire, a bladder-style tank can develop a slow leak through the valve, and the tank gradually loses its supply of air. When this happens, the pump has to work harder to maintain the pressure in the tank; it cycles on more frequently, and it can quickly wear out. To determine whether the tank needs charging, disconnect power from the pump and open a faucet to relieve pressure from the pipes and the tank. A pressure gauge attached to the tank's air valve shouldn't read much less than the pump's cut-in pressure. If it does, you need to add more compressed air; but first, you need to fix the leak, which is often due to a defective valve.
When a pressure tank becomes waterlogged, it fills with more water than it should, and because the pump can't maintain pressure, it cycles on more frequently than it should. This condition becomes noticeable at the faucets, where water pressure quickly drops -- sometimes to nothing. Tanks typically become waterlogged because the bladder becomes brittle and ruptures -- excessive chlorine in the water can cause this, as can minerals in the water. If the bladder ruptures and air escapes into the water, you'll probably notice bubbles in the water when you use a faucet.
Air bubbles in the water, when not accompanied by loss of pressure or frequent pump cycling, can indicate problems with the well pump or the well itself. The drop pipe to the pump could be leaking, or the water table could be low enough to partially uncover a submersible pump. Similarly, loss of pressure or frequent pump cycling may also indicate problems with the pressure pump. The impellers could be damaged, or an internal seal could have ruptured. The pressure switch on the pump, which tells it when to start and when to stop, could also be defective.