Hey Kevin,
       You said that coolant was leaking around the expansion tank -- I
thinks that says it all.  It is a sealed pressurized tank, no leaks should
occur.  If it is leaking coolant out when hot, it is sucking in air when it
cools -- both are bad.  You need to figure out if it is the tank, the cap, or
the coolant level sensor.  Get the engine hot, grab a rag, and watch for the
coolant.  Wipe off old stuff and look for fresh leaks.  I was getting leaks
from a cracked sensor, not to mention that the o-ring around the sensor was
as hard as a rock (aka: useless).  That's not too hard to diagnose, it sits
on top and coolant will pool up around it.  If you have a crack in the tank,
just replace it.  I had a cracked tank in my old Fiat X-1/9, only leaked when
the pressure started to build.  Didn't even notice it until I saw steam
shooting out of it once.  Not good.
       Check the tank, cap, and sensor first before going to the radiator and
heads.  It's easy and no mechanics will be hurt.

Bill Marshall
85 GL Tiico
Bradenton FL