Not sure if the Superformance and Factory Five use the same set up, but on mine the cap screws down and locks with a robber seal under it that prevents gas from sloshing out. Then the flip top cap on top has a rubber seal around it also. Did you check to see if you have a rubber seal and if it was cracked or there was some rough surface such as dirt or something that might keep it from sealing completely. It doesn't take much of an opening for gas to get through if it splashed up against something. About the same as hot
oil. It can go through a pin prick.
Ron