There are several factors that contribute to positive pressure. The vent valve requires more positive pressure to open than it requires negative pressure. (30-50 mbar positive versus 1-16 mbar vacuum.) The fuel cap also vents positive pressure when it exceeds 100-300 mbar. So evidently the designers believed that some positiive pressure was acceptable.

One way to check the vent line is to attach a rubber hose. Blow into the rubber hose to pressurize the tank. Then place the end of the hose in a container of water. If bubbles appear, you know the line is clear. The vent valve itself should offer very little resistance to air flow in the direction of the fuel tank; a bit more resistance in the opposite direction.