Fluid seeks level. Without being acted on by a vacuum/suction (pressure lower than atmospheric), or a positive pressure (higher than atmospheric), it will never go above level. The fuel line comes out of the tank and (almost always) goes above the level of the fuel in the tank. If the car is parked facing uphill, the pump is MUCH higher than the fuel level! Therefore, if there is no vacuum/suction on the pump inlet line (tank outlet), not a single drop of fuel would ever leave the tank. The tank vent allows air in to replace the volume of fuel that is drawn out. Otherwise it would create a vacuum in the tank and suction line. The air isn't rushing in and pushing the fuel out!!! Diaphragm pumps are capable of generating tremendous pressure both on the inlet and outlet. A diaphragm pump positively creates vacuum/suction on the inlet. A spring pushes the fuel out and limits maximum pressure.
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