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Discussion Starter · #1 ·
I know the header/ expansion tank is designed to allow coolant in and out. I also know that you are advised not to over-fill this header tank. Perhaps, in the grand scheme of things every engine may not be held accurately to the pressure/ temp.
I also think coolant can escape thru this header tank after it has been released from the radiator thru radiator cap into the tank.

Note, if heater is used often and hasn't been bled properly there could be a small volume of air in the system which will affect header tank level.

Please note the above is my theory of what happens .... I would like an expert to further explain.

Ps. My header tank often requires a top up after similar milage.
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