We have a shallow dug well about twenty feet deep and thirty feet from the house and the jet pump is in the basement. The pump and bladder tank are on the floor of basement at the same level. New foot valve installed and holds pressure in the line between the pump and foot valve. Well guy says lots of water in well.
The problem is the pump cavitates perodically and I it takes a lot of purging to clear the air. How does air get into the sealed system and how do I correct this.

Air gets inro the system on the suction side.
1) your well guy could be mistaken and the water level'sdown to the point that it sucks air until the water table builds back up. **OR**
2) There could be a leak in the suction line allowing atr to enter.
However, since the well holds pressure this is doubtful as pressure would bleed off if there were a leak in the pressure line.
Unless you have something open allowing air to enter the system I suspect a low water table. Regards, Tom