anyOne, I just went out to double check on my mower. The top of the fuel tank is at the centerline of the cylinder. There is a slight vacuum when the piston goes to the bottom of it's stroke (or toward the back of the mower in this case and it is a 4 cycle engine). It does pull air in thru the air filter and carb body but it also pulls fuel up to the carb. This action also puts available fuel into the carb for quick restarts after you have been mowing. When the mower sits for a few days, that available fuel evaporates and it takes two pushes on the button for the quick prime on the initial start.
I know that there are plenty of small engines and tractors that have had fuel tanks above the cyclinder. When the manufacturers were forced into the "safety" changes some years back, they had to redesign engines for quick and easy restarts since the engines had to stop as soon as the operator let go of the handle. If they didn't do it this way, the motor may have been able to continue running until it ran out of fuel from a tank above the cylinder and they would not have been able to pass safety tests.
There's something else to remember about this. Fuel evaporates. If you have a fuel tank with a small line from the bottom, a small seep and evaporation will pull a bit of a vacuum on a sealed tank. I know that with big usage you get enough air gurgling back into the tank to equalize pressure. But I have also seen it where a small leak allows little air to get in and it is the vacuum that forms in that situation that makes a cap hard to remove on a sealed tank.
A similar type of thing can happen on a totally sealed tank due to temperature variations. Take a 1/4 filled plastic pop bottle and seal the top with it's cap, then put it in the freezer. The thing will collapse. Put the same bottle in your hot car and it will about blow up. The air has it's own pressure in a sealed tank and this is easier to see with a gas like air than with a liquid. Either negative or positive pressure will make that cap hard to remove. Since you can get a lot of conditions going on in a sealed tank, making sure that you have a small vent to the open air is important, as you know.