Vince,
As you stated, the line needs to be open to "vent" the air space at the top of the tank. I imagine you're pop (blind) riveting the fitting on top of the tank. (I don't know how familiar you are with pop rivets) If so, the center of the pop rivets will be open (in some cases) after you complete the installation. This will let some fluid/vapor out. As for not pulling the gasket tight, pop rivets come in a variety of lengths, if you're using one too long it won't pull up tight.
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