actually, there are two types of radiator caps. A double seal cap is required for an over-flow catch tank. Non-catch tank setups only need a single seal cap.
Interesting, perhaps there's a difference between European and US radiator caps then. All the ones I've seen here have a primary seal which maintains the pressure in the rad and a secondary rubber seal in the top of the cap that ensures venting occurs via the overflow pipe only; otherwise you'd be at severe risk of being scalded by escaping steam when the motor boils and the primary seal vents.
Stuart.
