Originally Posted by JDCAVE
Sure there is:
Let's say my finger represents the filler cap and the pipette represents the head of water in a filler tube that is inside the water tank, the surface of the water in the bucket represents the water level in the water tank at atmospheric pressure. The air space inside the pipette is under a vacuum that is released when I remove my finger. If the "O" ring gasket is a good seal, and there is a filler tube that goes inside the tank, you can theoretically get a vacuum, even though the vent lines are functional and at atmospheric pressure.
The vent lines are functional.
Why would there be a filler tube inside the tank? If this was the case, everyone with that style of tank would have the same problem you have.
How about this: Take the hose off the tank and reach in and see if there is a filler tube or it the fitting just connects to the tank. My guess is, there is no tube.