Thanks for response.....I am a shady tree mechanic (used to do so for a living) and am stumped. Change plenty of fluid in my time. I have a shop manual for it...it only shows draining radiator. No, no previous overheating. I filled to max, then started tractor, ran till fluid warm and starting to overflow neck. The overflow tank did empty some after first event. I think you may be correct...may have to find a way to burp this baby.