[glang]

now Im confused! Cars have two radiators? Do you mean the interior heater heat exchanger or the maybe the aircon heat exchanger....
Anyway I think we're talking about the same thing: SVs HAVE OPEN TO ATMOSPHERE EXPANSION BOTTLES WHILE **SOME** CARS DONT.
You have a problem with your cooling system if coolant isnt being drawn back in as otherwise theres no point having a liquid level in the expansion tank. Its possible that your rad cap isnt sealing properly (the top seal not the inner one) or the hose from the rad to expansion has a small leak which doesnt let out coolant as theres no pressure but lets air in during cooling down when a vacuum is produced. Just for this reason I have a clear pipe fitted to my old car with this type of system so that I can see its always full of coolant