I have a similar question, what if the pump runs completely dry after 12-24hrs of operation? The pressure gauge on the sand filter will show about 30psi but no water is flowing at all. I can turn the pump off, turn the multiport valve to backwash, the pump will prime itself and backwash normally, then when I put the vavle back to filter everything works fine and the pressure goes back down to normal. About 12hrs later, the pump once again has no water in it and is running dry with a psi showing around 30!