If these guys were knowledgeable about wells/pumps etc. They wouldn't have tested the pipes with air. Air compresses, so it would take quite a while to notice a drop in pressure. Water on the other hand does not compress, so a drop of water can reduce the pressure from 50 to zero with a totally waterlogged tank.
I doubt that your footvalve being brass is the problem if its fairly new. What kind of jet do you have? A single pipe or a two pipe? If a single pipe, did they change the leathers?