Hello. When I have my skimmer valve and main drain valve open, the pump basket is full of water. When I try to run main drain only, water flows, but it struggles to maintain a full basket and air is introduced into the system. Obviously, my assumption is that air is getting in somewhere. But, why is air NOT getting it at all when the skimmer line is open too? Shouldn't air be getting in regardless if the main drain valve is involved? How does the issue go completely away when the skimmer valve is open? Secondly, I have checked all the unions above ground to see where air may be getting in and don't see any. Are there other causes of this issue? I.e. debris in and around the main drain (pool is still cloudy so I can't see if anything is down there) or air stuck in the line some how? I used a compressor to blow out the main drain for the winter. I use a little contraption to blow out the main drain until I see a good volume of air bubbles coming up. Could this have caused anything? Thank you very much for the help.