It sounds to me like your charcoal canister needs some love. Try
disconnecting the charcoal canister and see if this fixes it. If the vaccuum
goes away, then you've found your problem..... time to fix the charcoal
canister. I have heard of these getting clogged.
I've heard of people cutting them open and replacing the charcoal with
aquarium activated carbon. I don't remember for sure but there may be a
check valve in this filter housing?
This isn't a syncro right? Syncros have check valves on the fuel lines (They
are serious about anticipating roll overs...).

-Craig
'85GL turned WESTY
BOSTIG In the back
'87 SUNROOF Syncro
'90 Carat (still in someone elses driveway)

On Thu, Sep 29, 2011 at 10:18 AM, Steve Williams <
steve@williamsitconsulting.com> wrote:

> Hi,
>
> Obviously my gas tank reseal and replacement of gas lines is working
> great because now my gas tank builds up a vacuum while I'm driving.
>
> When I stop, at some point afterwards I get a large thunk as it pops
> back into shape.  If I take the gas cap off shortly after driving (eg:
> to put gas in it), a large hiss and thunk as the vacuum is broken.
>
> Going through the "Green Bible", about the only thing i can see that
> should be "venting" the gas tank is the charcoal cannister?  Does this
> sound right?
>
> Do these charcoal cannisters get plugged?
>
> I have no idea where to go from here :-(
>
> I guess it's a good problem to have compared to gasoline flowing onto
> the ground when filling the tank :-)
>
> Thanks!
> Steve Williams
> (the other one ;-)
>
> 1991 2.1 Westfalia
>