Tank's INC Vent/Rollover Valve

Dana67Dart said

There is nowhere outside the car that would be higher than the fuel inlet.

OEM runs a tube from the top of the filler neck down to inside the frame rail.

I don't see why you could not do the same. The rollover valve will need to be mounted in the correct orientation.

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The reason the OEM works is that the filler neck forms a "vacuum break." I you simply run a vent "up" then back down, it will form a siphon, if the tank is full enough

Only way the OEM vent can leak is if the tank is full enough that the level plus a bit of summer warmth will expand and force the fuel overboard. But as soon as it reaches equalibrium, the large filler neck forms a "break" and stops any siphon