Steve, one thing you could try is to remove your charcoal canister and
flip it upside down and tap it on a hard surface (your driveway) a few
times.  The charcoal can get clumped up with age and have a blockage
which can cause this problem.  Blow back into the tank (with the gas cap
removed) just to be sure there is nothing blocking that line and
reinstall the canister.  That should fix the problem.  It is located in
the rear wheel well on the driver's side in a syncro and the passenger
side on a 2wd Vanagon.  Just look in there you will see it.  It is not
hard to remove (large hose clamp holds it in place).  Hope this helps,

--
Thanks,
Ken Wilford
John 3:16
www.vanagain.com
Phone: 856-327-4936
Fax: 856-327-2242

On 9/29/2011 10:18 AM, Steve Williams 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
>