I've tried both ways. Sometimes good, sometimes not. YMMV, but I'm using 2$ getto grommet & polycarbonate tubes.Pressure causes leaking when the only escape path for that pressure is forcing liquid through the slot. Both ends are the same. The slot in the carto is all the way to one of them. You'd flip it so that the slot in the carto is at the top and fill it until just before liquid reaches the opening. Liquid in, air out through the polyfill.
I can't test this just yet until my tubes arrive, but I'd be shocked if it leaked.
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