I have the same problem when my solar heater starts up. A ton of air comes out. I also get air build up in the top of my sta-rite filter tank. So each week, I open the tiny screw top and let the air out until a jet of water shoots out. I don't have any visible leaking from the solar panels or from any other pipes, so I just assume the air is coming out of the water. I'm not sure why the solar panels keep filling with air, other than when it is turned of, it is probably under some negative pressure to let water flow into the pool and therefore air is getting into the system in the roof.