What kind of lost air recharge system do you have?
The most common type, 'air bleed', requires two check valves--a foot
valve on the submersible pump and check valve at the tank inlet. You
know you have an air bleed system if there's a tire valve between the
tank and well head. With this type system, if you just replace the foot
valve and don't have a good check valve at the tank inlet, your system
will slowly drain down to the level of the 'bleed hole' 5 to 10 ft. down