Where is my leak ..

[Poconos]

Something else I just thought of. You do mention the filter has a top mount valve. Ever look at how much the filter body expands or deforms when under pressure? Quite a bit. My Hayward uses a clamp ring to hold the valve assembly to the body and I could easily suspect that under pressure you have no leak but under a partial vacuum you may. Also remember, doesn't take much of a leak to let air in compared to water.
Al
Sidenote: That solar thing in my field has something like 30 joints. Black poly, bib connectors and hose clamps. I've pressure tested the sucker with compressed air up to 40-50 PSI as I remember and no leaks. Give it a little time, temperature swings and pressure cycles and something will always loosen and leak air under a vacuum.

[mwsmith2]

With the heated water in the panels I think it's more likely that you have outgassing in the water than an air leak.