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.