I've got an 88 Westy that I've had for about 3 months.  I'm the
second owner.  It's got 120,000 miles on the clock.  Got all service
records from previous owner and it had a head gasket job done at
around 80,000.

So I just drove it locally (with no problems) until two weeks ago.
Then we made our first road trip in it for a round trip of around 600
miles.  About 100 miles from home, the red overheat light starts
blinking.  But the needle is only halfway, like pointing at the light.  I
pulled over, shut it down, and looked for a busted hose or broken
belt.  But there is no sign of a problem, no leak.  Started it up to
see if it squirted anywhere, but no leak, belt is turning water pump,
and now the light is off.  So I thought it was a fluke and went on.

Not long after, the light comes on again.  This time, I notice the
reservoir is full - the one behind the license plate.  The other
coolant tank has an air pocket.  So I get a water bottle, slowly
loosen the cap, let out the pressure and top it off.  Then I go on the
rest of the way (200 more miles) with no problem.

This same thing happened once on the way back.  But the needle
never gets any higher than pointing at the light.  So I'm wondering
what's going on.  I'm no expert, but I think I suspect what's going
on.  Could the head gasket be slowly leaking combustion gases
into the water jacket ?  I don't know if this is possible since I don't
know if coolant pressures would be higher than combustion
chamber pressures (I suspect not).

Anyway, I wonder if anyone's seen this before.  I know that the
waterboxer head gasket can be a problem with leaking coolant, but
this is a little different.  If it is the problem, should I expect to need
new heads or just a new gasket ?

Thanks!