OK, as you all know I'm not an exactly the sharpest of knives. But this doesn't make sense. If a lack of a vacuum would cause leakage, wouldn't a full carto leak. Isn't there a inner tubing in the cart that keeps the full cart from leaking? I think the only way it could leak is not a lack of a vacuum, but either the tank is so full that it is higher than the inner tube or cut/punched thru the tube during set up.
BTW I took the top off my tank and let it sit for an hour without any leakage.
A full carto does leak when filling them until the juise is absorbed into the matting. That's why I always use a .... plug carto condom when I fill cartos. The CE2 has no matting so there's nothing to catch the juice.
Yours didn't leak because it has matting absorbing the juice.