Hi Gordon,
            There is a 12mm plastic vent pipe above the top of the tank.
This pipe lets air from the tank as you fill up. It is especially needed
as fuel covers the big fill pipe on the tank right side.  Air from the
tank comes up this vent pipe on the top left, goes back into the tank on
the top right then exits the tank on the top right through another 12mm
vent pipe attached to the filler pipe.  Keeping the van left side a bit
lower than the right as you fill up will help. If the left side is
higher than the right it is very slow to impossible to get fuel into the
top of the left side.
       The wheel well air tanks, charcoal canister and the rest of the
pluming on top of the tank is used to vent the fuel tank after the fuel
cap is installed.
Midwesty

On 1/13/2017 9:40 AM, Gordon wrote:
> I've been having a harder time filling my 89 Westy's gas tank more and more. Something is blocking or causing back pressure such that the gas station nozzle is made to stop. I am forced to slowly feed the tank an ounce at a time and it takes forever with much spill over.  I've tried several stations and it isn't them, I'm afraid.
>
> I've heard of some filter? Or charcoal? In the way there?  Anyone know what part may be the culprit?
>
> Thanks,
> - g