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The lower tank doesn't need to be air tight. It's just a reservoir to catch any coolant that gets pushed out of the upper tank which is part of the pressurized system if the pressure gets above the cap pressure rating. As the system cools and contracts it will pull coolant from the lower tank like drinking with a straw. Best I can explain it. Just top off the upper tank when the engine is cold and make sure the bottom tank is between the lines.
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