first I'd solve why the coolant ain't being drawn back into the rad: bad recovery cap or leak in the hose or container (above coolant level being burped out) that is keeping vacuum from drawing the coolant back to the rad when the system cools off. IF (I ain't sure) recovery tanks are vented (very well might be as the hose comes in thru the bottom) then the hot coolant could simply be evaporating out of it as it sits.