[Aussie Mike]

Good point Ben. The doors are not a perfect seal. Just metal on metal. Enough to stop the big surges of fuel but fuel will leak past them on regular driving.

There will also be a couple of small holes near the bottom of the surge tank. This will allow the surge tank to fill when filling the tank otherwise it wont fill till the fuel level reaches the level of the trap doors. You would quite a bit of fuel in the tank to get the car started otherwise. Also when the fuel level gets low i.e below the level of the trap doors the small holes will it will allow some fuel into the surge tank. Otherwise it may be possible for the pump to drain all of the fuel out of the surge tank and run out of fuel with 5L or 10L still in the tank.

Cheers