I just moved into a house with a shallow well, after finally getting it primed and flowing; it doesn't put out much water. It can build enough pressure to turn off the pressure switch, but after 5 minutes loses pressure and switch kicks pump back on. If I turn it off it loses prime altogether. It's all old galvanized, I don't see a check valve but that doesn't explain the lack of continous pressure when the pump is running. any help?