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Exactly, but the way the system works is that when you start with a brimmed radiator and an expansion tank half full, as coolant heats it expands and pushes the extra into the expansion bottle, so you need a bit of room for this. As the engine cools the coolant contacts again and the vacuum draws back the extra from the expansion bottle again so when cold it should be back where it started. As it's a sealed system there should never be any loss of coolant so once you have filled the rad to the brim and the expansion bottle half way, should there be a slight loss (nothing's perfect) then the lo

What a lovely forum this is! Well done Brian, that's a very nice gesture (not that I would expect anything else!).