Read through a similar post and I determined that I clearly have a suction side leak, with air bubbling out of all of the returns.
So, somewhere between the skimmer and the impeller, air is getting into the system. I've been pretty systematic and figured out where it isn't by process of elimination - the O-ring on the pump strainer basket lid is new & well lubed, drain plug lubed & tightened, etc.
My only remaining guess is the stem from one or both of the gate valves that control the flow from the 2 skimmers. I've tightened the gland nuts, even though they didn't show any signs of leakage, pump on or pump off. One of the valves releases a little bit of water (maybe 2 tbs.) when I shut the pump off. I guess I'm reluctent to believe that such a small leak could be the cause of so much air into the system.
Think I need to repack the stem with new plumbers tape?
Any other thoughs?