As others have said, it could just be the O-ring between the valve body
and the core.  When I got my GL, the rear heater smelled of antifreeze
whenever I used it.  I removed it, pressurized it with air and dunked it
in a bucket of water.  I found bubbles from the O-ring area but none on
the core itself.  So, I cleaned it up, installed a new O-ring, and
repressurized it.  An hour later I checked and it was still pressurized.
So I thought I was good to go and reinstalled the heater.  No luck,
still smelled.

I forked over the $$ for a new rear core and that cured it.

I think the core can pass a cold pressure test but still leak a bit when
they heat up.  It only takes a tiny leak to create that unmistakable
smell.  The rear heater cores are well known for developing leaks; I
hope the replacements are better than the originals in that regard.

Allan
--
1991 Vanagon GL

Florian Speier <groups.florian@GMAIL.COM> writes:

> detected some nasty rear heater leakage in my westy.... first I thought the
> core was bad, and it does have a little rust on the bottom, but now I bench
> pressure tested it somewhat and it seems for sure the seal next to the valve
> is gone,  but saw no leakage from the core. of course a 5 min test does not
> resemble a 1000 mile drive. makes me think whether the rust on the core is
> just of the coolant running down and pooling up in the plastic case the core
> sits in......
> so do the heater cores typically go bad at all? am I safe just replacing the
> valve?
>
> thanks everyone
> flo
>