Is the skimmer door stuck partially closed, where it lets the skimmer fill with water until the pump starts and then the skimmer runs dry?
We thought we had a suction side leak on the skimmer line because the pump would not prime if the skimmer line was open, but would when only the main drain was open. The skimmer door had a small piece of debris stuck in it that caused the door to be stuck in the almost closed position. This allowed the skimmer to stay full of water when the pump was off, so it wasn't noticed at first that the skimmer was running dry.
I didn't read the entire post, so this may have been discussed, but it may be worth checking.
We thought we had a suction side leak on the skimmer line because the pump would not prime if the skimmer line was open, but would when only the main drain was open. The skimmer door had a small piece of debris stuck in it that caused the door to be stuck in the almost closed position. This allowed the skimmer to stay full of water when the pump was off, so it wasn't noticed at first that the skimmer was running dry.
I didn't read the entire post, so this may have been discussed, but it may be worth checking.