Problem using Airlift to pull vacuum

I replaced the water pump on my wife's 2010 Prius, and now I'm all set to refill the cooling system. I purchased an Airlift vacuum system to speed up the refill and help eliminate air bubbles. The coolant tank refill neck narrows to a very small opening just past the mouth of the neck, which prevents me pushing the funnel shaped Airlift adapter far enough into the neck to get a good seal without the adapter moving around. But I found if I combined the funnel adapter with one of the other smaller adapters, I could get what looks like a decent seal, and is reasonably stable.

So, with the Venturi valve secured to the Airlift body, and air hose attached, and overflow tube and bottle ready, it looks ready to go. I turn on the air and I hear a high pitched whine of air movement through the Venturi valve (which I think is normal when the Airlift valve is still closed), I open the valve, but no vacuum develops. The only noise is air flowing into the overflow bottle (but now without the whine). :-(

I have a feeling I'm forgetting something, but I can't figure out what. I double-checked the radiator drain cock to be sure it's closed tight. I suppose the Airlift may not have a good enough seal to the tank, but it looks good enough to least draw a slight vacuum. It's almost like the Venturi valve is broke, but I doubt that since it probably has no moving parts. I double-checked the Airlift body valve to ensure it appears to be opening and closing properly. It looks like the water pump is well mated to the engine all the way around.

Any ideas about what I might be doing wrong? Does anyone have a good tip on how to get a good, stable seal between the Airlift body and the tank?