The radiator cap was the most likley problem . When the cooling system is hot it will vent excess coolant through the cap into the overflow bottle and when the system cools down it draws the coolant back into the system from the overflow bottle. Its possible that there is a blockage in the hose from the expansion tank to the overflow bottle. If the system is not venting properly even a new hose will collapse.

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Mercedes Benz Master Tech - Retired