Aztek airbrush question

I think you have the cup upside down. The black section is a top for the cup to help prevent spills when using. The bottom slot fits into the AB, and the air supply is through the AB, not the cup.

As far as I remember, those were the first cups, before the white models which are one piece only; you'll notice that paint gets out of the bottom of the white cup; let's say it's gravity feed; the grey and black cups work differently : paint goes outside the inside cup into a thin "tunnel" along the outside cup and into the outgoing tube to the airbrush; I should call it semi-aspiration feed; I don't remember of these cups being better or worse than the white ones; maybe they were intended to regulate paint flow, but I never saw big difference; apart from the extra cleaning, which is very little, you can use them with confidence.