Many, maybe even most or all, 9" diffusers have a check valve ACTION as part of their design. My Vertex units don't have a check valve per se, but when air is shut off to the diffuser the membrane settles down over the slightly raised center, and acts like a check valve to inhibit water entering the airline.

I don't run check valves on mine, but perhaps others do.

"Forget pounds and ounces, I'm figuring displacement!"

If we accept that: MBG(+)FGSF(=)HBG(F1)
And we surmise that: BG(>)HBG(F1) while GSF(<)HBG(F1)
Would it hold true that: HBG(F1)(+)AM500(x)q.d.(=)1.5lbGRWT?
PB answer: It depends.