Yes it sounds like it had air in the heater circuit which you flushed
out when you opened the heater valve to 100%.  The air would have
probably then gone to the expansion tank and been bled out over the
next cool down/heat up cycle.  I would just check for air bubbles at
the radiator bleeder, there might be  a little air stuck up
there.  There's a lot of talk about troubles with bleeding the
vanagon cooling system but on my 2.1 it goes really easy.
Edward

At 01:57 PM 6/15/2011, Wes Neuenschwander wrote:
>I recently had the coolant system flushed and coolant replaced in my '91
>Westy at a local independent VW service place.  I have about 15,000 miles on
>my rebuilt (GoWesty 2.3 liter) engine.  When I got the vehicle back, the
>coolant in the reservoir (non-pressurized tank) was at the 'full' line.
>After driving the vehicle about 100 miles, I noticed the heater didn't seem
>to be heating fully.  After I opened the heater valve 100% for a few miles,
>the heater function seemed to be restored, providing well modulated heating
>over the full range of the heater valve settings.  Engine temperature - or
>rather coolant temperature - was normal the entire trip (consistently
>mid-point on the coolant temp gauge, just as before the coolant flush).
>
>However when I got home from the trip (about 500 miles, including some NW
>Washington mountain passes, I noticed that the reservoir was empty (didn't
>check the pressurized tank, alas).  I refilled the reservoir and after
>running the engine a few times during short day trips around town, both the
>reservoir and pressurized tank levels are normal (pressurized tank is full
>and the reservoir is at the mid-full mark).
>
>There is no evidence of leaking coolant - anywhere.  My guess is that the
>air was not fully purged from the coolant system following the flush/refill
>operation (perhaps, at least in part, due to air trapped in the heater lines
>and/or heater core).
>
>Two questions:
>
>1)      What do I need to do (if anything) to ensure the system is fully
>purged now?
>2)      What possible adverse effects might there have been from having an
>incompletely purged coolant system on a drive like this (including some
>full-throttle periods over the mountain passes)?  In particular, of course,
>is it likely that I might have damaged or otherwise compromised those
>infamous waterboxer cylinder heads??
>
>Also, a corollary question:  I clearly specified that the replacement
>antifreeze must be Volkswagen brand only (in the past, I've had shops use
>'house brand' coolants - i.e. whatever the local tech/guru thought was
>best).  How can I verify that the antifreeze used was genuine VW and not
>some off-brand?
>
>-Wes