I have an in ground pool with a cartridge filter system and rooftop 7 panel solar heater. As of late, air has somehow been entering the system resulting in decreased water flow and tiny air bubbles coming from the outlets. When I open the filter, remove the element, replace it and reseal the filter, the water flow is back to normal for 2 to 3 days and the problem reappears. I have no water loss and therefore no leaks. The filter o-rings are generously lubed. I have been thinking that the rooftop relief valve may be the culprit. Any help would be greatly appreciated.