[77newell]

If the radiator is not full and the overflow tank has fluid in it there is a problem either with your radiator cap or the line to the overflow tank isn't air tight (either at a connection or the line itself). Whenever the radiator level gets lower than absolutely full to the brim when cooling after shutdown the coolant in the overflow tank is supposed to be sucked into the radiator which in your case seems not to happen. If the system is working properly you wouldn't need to check the sight glass, just check the overflow tank when cold to see that it has fluid in it.