Ken, I think you'll need some holes in the rubber gasket. The way I'd do it is more or less a copy of the B&S tank cap: drill a hole in the center of the cap, and cut an oil-resistant rubber disk that just fits inside the cap. Punch four holes in the rubber disk, in a pattern so none of them is close to the hole in the cap. When a vacuum starts to form in the tank it will pull the rubber disk away from the cap, allowing air to flow through the holes in the cap and the disk.
