I have started another thread here
http://www.bristolcars.info/forums/r...-pressure.html to discuss the topic of radiator caps and why we pressurise the cooling system.
Another thing just occurred to me however given Greg's revelation that the 408 has an integral expansion tank. A modern radiator cap has TWO valves - one pressure activated valve to let coolant out (usually into an expansion tank) once the pressure exceeds the cap's rating. It also has a vacuum operated valve which allows coolant back in from the expansion tank as the system cools. The coolant usually flows out and back in through a pipe in the filler neck where the radiator cap sits. But how does it work on a radiator that has an "integral" expansion tank?