If the aircraft does not have a fuel jettison system, then the refuel lines are emptied as soon as refuelling completed. The refuel gallery is only used for refuelling and is drained through NRVs into the tanks as soon as the pressure is off. Some aircraft use the refuel gallery as the fuel jettison manifold so the cap is required As a back up. The coupling itself id selfsealing. The B747 has mechanical shut off valves so the fuel cap is the third stage of safety!