Thanks Pathfinder but I think I found my problem.
If I had added more info, like the part that I live in a cold climate(Michigan), you would have probably figured it out too.
Using my infinite powers of deduction (HA!), I reasoned that I obviously have an air leak somewhere in the return (suction) side. Since I had no air showing in the pump sight glass under normal operation, I reasoned that it must be caused only when the line is under a "load". In other words - caused by the added suction from 30' of hose lifting water from an 8 ft depth.
Obviously any leak would be below where I attach my vacuum plate to the skimmer. So it was either in the return line itself, or in my skimmer basket housing (Pardon me if I don't have the correct nomenclature).
As an experiment, I stuck the vacuum hose directly into the return hole at the bottom of the housing - bypassing the skimmer basket. The pool vacuum worked great - not even a hiccup.
Therefore, I reason that I have a crack or hole in my skimmer housing that only leaks under vacuum - amazingly through the underground dirt that surrounds it, and the concrete sidewalk over it (actually the dirt is mostly sand - so I guess its possible).
I looked but couldn't find anything that looked like a crack or void but I applied some epoxy pool putty to some suspicious areas. Will see if that works. It's hardening as I write this.
When I winterize the pool, I always plug the return line hole in the skimmer housing and pour in some RV type anti freeze. I'm thinking that something happened last winter - my anti freeze failed or leaked or got diluted and some water got in my skimmer housing - froze - and caused a crack somewhere.
Don't you love it when people figure out their own problems?