I have an Addison heat/ac system with a shallow well pump. The pump would hold its prime during the day but when the ac ran less at night the pump lost its prime and had to be manually primed. I replaced the pump and it continued to do the same thing. I replaced the check valve and it still will not hold its prime when the ac recycles. I have found no leaks at all. The AC does not seem to be running long enough to allow the pump to prime itself. It has a safety shutoff to prevent damage. Any suggestions will be appreciated. Leon