I've got a 84 Westy. About a month ago, the temp warning light came on
suddenly and started flashing, even though the van had only been running a
few minutes. I checked the rear coolant reservoir and the level was way too
high. (That's the overflow reservoir visible behind the license plate.)

I took it in to my local VW shop and they replaced the front reservoir and
sending unit. It was ok for a week or so, then exhibited the same problem.
They said the raditor was probably clogged, so I had them replace it ($400).

Again, ok for a couple weeks, then the problem returned (warning light
coming on suddenly, too much coolant in the rear). Today they said they
think that pressure is building up in the system and forcing coolant into
the rear. They said the only place this could be coming from is a leaky head
gasket. They quoted me $800 for the job, assuming they can reuse the heads.
More like $1800 if they have to replace the heads. Yikes.

Does their diagnosis make sense? Is there any other explanation I should
explore?

Thanks for your help,
Ken