You're losing coolant somewhere if the reservoir is being emptied on cooldown. Ideally, it should stay about half full and the radiator should stay entirely full- - - -no air at all in the top tank.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!