Nothing more than the fact that too much pressure will push the gas out at
just every possible fitting.  Perhaps that's why the car had leaks here and
there ; fuel filter, fuel pump etc.
Is there any possibility that the pump might have been installed the other
way around, or perhaps crossing pipes and vent pipes.  Is the pump known to
be good ? Perhaps it is worth double-checking...
al.

In rec.autos.makers.vw.watercooled, Chris wrote:

Somebody here will have an idea beyond mine-- to get the right cap
at a dealer.

But you need to understand why your leaky cap is passing liquid
instead of just vapor. Did you crawl under the car to see if the
tank looks like it is badly dented?

On 23 Apr 2007 10:45:22 -0700, in rec.autos.makers.vw.watercooled
(message <1177350322.426183.86980@o5g2000hsb.googlegroups.com>), Chris
<williamschris@uwstout.edu> wrote:

Checked which lines?

Living in Southern Ontario, I replaced more than one of these gas
tanks, and I will tell you that there is a number of additional
vent lines coming off the top of the tank.  

You mention in another post that the car was parked for three years.
It could be that the vapour-control system from the fuel supply side is
clogged by various gunk that built up, or that it got kinked or
otherwise damaged by an animal that was up in there (mice are small and
invasive).  I'd consider completely draining the tank, dropping it, and
finding out how many of the little hoses are connected, where, and
whether the lines are clear.  Working under there is a bit of a PITA,
but as long as you're not all rusted out underneath, the job is by no
means impossible.

In any case, if the pressure is so high in the tank that it can push
gasoline out the top of the filler tube, then it's much higher than it
ought to be (from memory, that would mean it was raising the fuel level
in the neck of the filler more than a foot, which is a fair amount of
pressure).  No gas cap will be able to hold it back, because it'd be
dangerous to keep such a volatile fuel under that much pressure.

One other thing -- I'd be incredibly careful where you park this car,
&c., while this is going on.  (Well, actually, _I'd_ drain the tank and
stop using it until I'd figured out what the problem is, but I have an
aversion to going up in balls of flame.)  In spite of its ubiquity,
gasoline (especially when spilled and dripping down the back of the
car) is very dangerous.  One clever twerp with a burning ember could
make your day a whole lot less happy.

A

In rec.autos.makers.vw.watercooled, Chris wrote:

A fire extinguisher might be more to the point. I would avoid taking
passengers until the problem is fixed.

It would seem to me that the tank is probably distorted in some way.
If it is not dented on the bottom, perhaps it sucked in from the top
due to a vacuum at some previous time.

I expect you will get some expert opinions by midnight.

My  > guess<  is  the air supply hose on the charcoal filter is plugged
keeping the fuel tank system under pressure when fuel expands
either throughout the day or just to ambient temperatures from
the temp. the fuel was when stored in the tank underground at the station.
There is a Gravity Vent Valve in behind the filler tube at the level of the cap
that could be stuck shut  maybe ---   or blocked lines to the expansion tank
or blocked lines to the charcoal filter . Does your manual have a picture
of the fuel supply componets with the tank , expansion tank , charcoal filter
pump and all? You can get some since of he vapor recovery system from
that.