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Check the small-diameter hose between the rad and the overflow bottle. If the hose is not completely sealed, the coolant in the bottle will not be "sucked" back into the rad as the engine cools. Therefore, the bottle will continue to fill up as you add coolant to the rad. Maybe the hose has a small slit or it is not sealing at the connection points.
So what happens is you top off the rad, the engine heats the coolant, coolant flows to the overflow bottle as it expands, as the engine cools and tries to suck the coolant back into the rad it cannot apply a vacuum to the hose because there is an air leak in the small hose. Just a possibility. I've seen it before.
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