Doing my first opening myself here and had a head scratcher. I took the plug out of one of my returns (the jets in the pool - that's a return, right?) and on the first one there was a major vacuum and a bunch of water immediately started getting sucked in., like the pump was running backwards (haven't even started it yet).
When I did the other one I got a major air bubble.
As a side note, I get that kind of behavior regularly in the morning startup.
Does that indicate there's maybe a leak in the second line somewhere? And, if so, how the heck would I find and fix it?