I have an in-ground pool with the Hayward self priming pump and a Pantera sand filter. Everything has been working fine until this morning. The pump fills up and maintains perfect pressure for a few seconds then completely drains and refills. It does this constantly. I do not have a a weir door on the skimmer and don't think there is blockage or leakage because it does reach full pressure before collapsing. Does anyone know the answer to this Mystery?