The last pic you posted of the skimmer with both lines open is the problem. It's pulling the water in to fast and creating a vortex that is introducing air.
I'm to lazy to research it but it looks like your pump is a 2hp Pentair Whisperflo? If so then you may be oversized for only the one line. If it's 2hp single speed then you may need to start saving for a smaller pump or at least a variable speed one.
They may make a vortex eliminator that you can put into your skimmer under the basket????
Hi Daltex,
Thanks for your reply. Let's back it up.
System is fine with the right line opened (no air in the pump) but the left line goes and automatically the pump fills with air, not enough that the system stalls but there is air in the pump.
If we had the vortex issue should it not occur with either line opened? I am using a Pentair WhisperFlo 1HP.
Will post pics on new reply