I have a friend who has an '87 Weekender that has a coolant problem.  He is
currently seeing dramatic level changes in his overflow tank.  Coolant
filling back into the tank (other than normal hot/cold
expansion/contraction) to me indicates a leaking head gasket where some
combustion gasses are forcing into the coolant system and displacing the
antifreeze.  This would eventually overpressure the expansion tank and push
coolant into the overflow tank.  Does this sound correct?  Are there any
other possible explanations?  He has already put in a new radiator and the
pressure cap was replaced.  I think he has observed some over-heating on
longer trips and when he stops and checks the level, it is overfull.

                                                             Jay