Sorry guys, I was taking a nap. Old age will do that to ya.
The tank doesn't do a vacuum form collapse (at first) but it does a series of minor fluctuations with each run-shut down cycle until the weakened metal finally splits. The plastic is much tougher than the metal and if you're lucky the venting diaphragms will burst first. This only occurs IF you make the tank into a closed vacuum system which neither of you have.
The slight "whoosh" is semi-normal in the summer time. I don't remember which thread I explained the "burping" action of the diaphragms in but I did. The system only vents when it is running so if the outside temperature rises as the car sits the gas expands to create the "whoosh".
Luna, you only have a partial closed system. Your vent tank is not connected to the active loop so while you are not making a vacuum reservoir out of your tank you are not venting the fumes through the intake system. When your car is running you have made the engine block into a vacuum canister. The next time your car is running, pull the dipstick and place your finger over the tube. I'll bet you find vacuum.
Both of you guys can complete the venting system by "Tee-ing" into the big hose but you need to create an escape vent or else the vacuum from the system will use the tank as a reservoir and create the collapse-expand sequence.
It can be as simple as an in-line fitting with a filter. I'm trying to figure a way to make it the least complicated, most effective and unobtrusive.