I've had the same problem.  Most likely your charcoal canister is plugged;
you can test this by disconnecting the line to it.

You can replace it with a new or used one, or you can vent the tank at the
cap by removing the lock core and taking out the rubber o-ring on its end
that seals the core cylinder. Grease it up well before reinserting the core
and you won't have any problems with water entering the tank, it's a snug
fit.

The former is the correct, expensive fix, the latter is quick and dirty, but
free.

Starting in the late sixties US safety laws required that no gas can leak
out of an overturned car, and no vapors be vented into the atmosphere, hence
the gravity valves and sealed tanks with venting through charcoal.

Stuart MacMillan
'84 Westy w/2.1

Grungy wrote:

Now that I have my gas tank sealed up properly, it is displaying the
old symptoms of sucking itself flat with excessive vacuum.

All sorts of crumpling sounds while driving and a big "whoosh" when
opening the gas cap, accompanied by un-crumpling sounds.