Just when everything seems to be sorted out, it isn't.  Son's '87 is losing
over a quart of coolant every 400 miles, and I can't find any leaks.  Rear
heater has been removed, and there is no odor from the front heater.  No
drips anywhere-engine plumbing, heads, transmission cooler, radiator, or
front heater valve.  No evidence of coolant mixing with either the
transmission fluid or engine oil, and that much coolant would certainly foam
oil.

I'm wondering if it's burning coolant, but in my experience failed sealing
rings in Vanagons lead to over pressurization of the system, and that's not
happening. It also runs great. I sometimes notice a faint smell of burning
coolant at the engine compartment vents, but I can't find anything dripping
on the exhaust when the system is pressurized.

Would investing in a UV leak detection kit tell me anything?  It would seem
losing this much coolant would show up somewhere.

Any ideas?

Stuart