Air in pool return lines?

After my pump switches off, I can hear and see, through a backflow valve, air filling the lines on the return side of the pump. The suction side and pump have no air in them. All rings on the suction side, at the Jandy valves have been replaced. It seems to start in the backflow valve area to the spa. This valve has had all rings replaced as well. When the pump kicks on the next morning, the pool and spa are filled with large amounts of air. The Jandy valve on the pool side has been rebuilt as well. Any ideas?