trans overflows after short time driving

Looking at buying a truck a guy has for sale...it has been pretty much rebuilt front to back. The guy is selling cause he has run out of patience and time to work on it.

The main thing wrong with it he says is that after a couple miles of driving it the trans seems to overflow out of the top...fluid looks bubbley if you pull the dipstick....says he has pulled the trans a couple of times...replaced the radiator....not sure what else to do and just wants to get rid of it......any ideas???

"Overflow out of the top" as in dipstick tube or bellhousing?

Bubbly fluid means it's aerated from being overfull, overheated or an internal leak.

Replaced the radiator- is it replaced with a radiator with a transmission cooler or does it have and auxiliary cooler. Has to have a trans cooler.

I wonder if he is filling it park or with the engine off, and therefor overfilling it. It shouldn't flow out of the dipstick tube, though since the case vent is a lower spot.

it's possible someone might have swapped the dipstick or tube also. could be "filling" it, with a different stick. (if they're available in different lengths ) foaming/bubbling is the sign of overfilled.