Magnesium's sweet.
I gave the whole skimmer a clean, ran the pump in the sink for half an hour or so with the venturi hose sucking in water, had a play with the adjustment nozzle while it was in there and sucking air. I didn't realize the dramatic difference in the amount of water it pumps with the nozzle wide open vs mostly closed. Flap was perfectly fine, btw. So, after I reassembled everything I thought I'd try finding where the flow of the pump(By watching the level of the water in the outlet) starts to dramatically drop off and try adjusting the air valve around that point. Seems that I get the most bubbles setting it just slightly open past where the water flow starts to nosedive. That's about the 1:30 position. We'll see how it goes now...