Hunter........It took me awhile, but I finally had a minute to test the gas cap. I unscrewed the gas cap, wiped most of the grime off, made sure no one was looking, and then I wrapped my knarley lips on that cap and tried sucking air into my lungs. Im guessing that around 10-15psi, I got a wierd "duck" noise. Quakkkkkkkkkkkk, quakkkkkkkkkkk! Seriously, it sounded like a sick duck or wounded turkey. So, it does allow air to come in at a certain pressure point.

I tried blowing air into it, like I was blowing an old brass trumpet, but got no air going out the cap.

So, looks like the cap is working as it should. Strange about that hissing sound I heard weeks prior......maybe the internal vacuum was not quite at that 10-15psi point to allow air in.

I might take that darn cap with me this fall when I go duck hunting......might do the trick.
Maybe your Montero cap will do better calling wabbits.

Let us know if you are successful with your new found duck call :)
Sounds like your cap is working. Perhaps, as you say, the vacuum in the tank needs to be below certain level before it lets air in. I suppose it is better than developing excessive positive pressure.