You probably have a bad seal at the top of one or more cylinder where it
meets the head, inside the water jacket. This is part of the head gasket
on most engines but it is a separate piece on these. The problem was
almost certainly there before you bought it. The "steam" you think is
bubbling in the coolant tank is really combustion gases leaking past the
sealing ring. This high pressure gas leaks into the cooling system and
forces coolant out the overflow tank. Another possible source for the
gases getting into the cooling system is a cracked head. Pretty common
problem and many people sell the things rather than risk the worst case
$2000 head reseal or replacement. Only $800 if you are lucky, $500 more
for each head if you are not.

Mark