Yes, I just went through the same thing.  Undercoating made it difficult to
remove too.  Use a wide blade to pry off the  tab on the tank that's on the
stud, it has to come off  first.  It can be a bear, just work carefully.  On
the driver's side the bump didn't line up with the recess in the body and
was much easier to get out.

I assume you are dropping this to replace the grommet.  If your grommets on
the vent valves are nice and firm, not gooey to the point you can gently
pull out the valve, leave them alone.  You'll break the valve stem if you
try to remove one from a sound, hard grommet.  If you do need to replace it,
use a heat gun to soften the hard plastic line before trying to remove it
from the valve if you do decide to remove it.  It is a bit easier to work on
with out from under the car.  Broke my stem right off, and had to get one
out of a junkyard, but GoWesty has them for 35 bucks or so (!!), but at
least they have them .  Lube with WD-40 to insert the valve into the grommet
after it's installed in the tank.

Stuart
'85 Westy

Larry A. wrote:

I'm not sure if you are familiar with the shape of the top of an expansion
tank.  There is sort of a bump which fits into a sort of socket at the top
of the wheel well.  This, along with the tab that goes over the stud, holds
the thing up.  Two studs would have been more secure and practical IMHO, but
oh well...
   You have to wiggle it vigorously and pry gently at the tab that's on the
stud.  Once free from the stud, the tank will come out of the socket and be
free.  I can't swear that the filler tube won't interfere because I've
always had it out anyway when removing the exp tanks.