You are talking about a coolant recovery tank. No it cannot be sealed. In order for the coolant to be drawn back into the radiator when the cooling system has cooled off, the recovery tank must be vented. Otherwise the radiator would simply pull a vacuum on the recovery tank and not draw much if any coolant back into the radiator. The radiator cap controls all that. It has two valves on it. One that the large spring in the center controls to open and let coolant into the overflow tank and the second smaller valve in the center that opens when the system cools off and allows coolant to be drawn back into the radiator.
