Gas tank leak

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Another thought — check for a plugged vent line (the small hose that's connected to the side of the fill tube).

If the cap is sealed (it should be), pressure in the air at the top of the tank can only escape through the vent hose. If that's plugged (or kinked maybe), the pressure would be significant on a hot day.

Each of those sender mounting screws should have a fiber washer under the head to stop gas from weeping past the screw head.  If they’re not there you can get them at an ACE hardware store or local DIY store or just order a sender gasket set with washers from a place like CB Performance or CIP1 VW parts - Most VW/Speedster senders use the same gaskets as a kit.  Order a set for a sender for a 1970 VW sedan.  
Personally, I use a cork gasket between the tank and the sender and then those fiber washers between the sender and the mounting screw heads.  The rubber gaskets may work fine, but i was just playing with my sender and found a cork gasket in there so there you go.  It’s worked for 20 years.

Before doing any gasket upgrades you will have to drain off some of the gas in the tank to get below the sender level.  You can drain it into a 5 gallon jerry can or just drive the car around for a couple of hours - Maybe the Jerry can is easier?  Once the gas level is below the sender you can remove it with no spillage and install the new gaskets.  

Good luck with the repairs!

I think my screw gaskets/washers are nylon.  One is copper where the ground attaches.

plugged vent line sounds most plausible

@barncobob posted:

plugged vent line sounds most plausible

As does an over-full gas tank.

The stock VDO sender seals (older style) are either paper or cork and can dry out over the years and leak.  The new EMPI (and other Chinese version) seals are rubber but not gasoline safe and they bloat and deteriorate and leak.  The new VDO seals are also rubber and seem to hold up under fuel OK.  I am NOT a big fan of sealing up gaskets that are not meant to be sealed, but sometimes you just have to do what is needed to prevent leaks.  We use aviation permatex on them which seems to help a lot.

Again, Carey nailed. I had one gas tank that would seem to leak after a few month no matter what I did. Tried cork, tried rubber, always seemed to start leaking again after some time.  I put on permatex, never had the problem again.
-=theron

third that.  Permatex aviation Form-A-Gasket is really great stuff.

You may need Dynamite to get it off 20 years from now, but in the meantime it'll never leak.

Oh!.....   and don't get any onyah!

that dark brown, nasty smelling sealer...no chemicals got it off ur hands..it had to be sanded off or let time do its thing..

Gordon your solution to lower the fuel level by driving around for two hours would empty my tank.

Thanks for all the replies and advice.

My plan was to reduce gas in the tank and replace the gasket this weekend.

I checked on the leak after work today and it seemed to be worse.  Good thing I positioned those Bounty paper towels carefully as they absorbed all the leaking gas.  Just like in the commercial absorbing juice or milk or whatever.

Anyway, I couldn't wait for the weekend, I had to do something tonight.

I let the Speedster idle for just a  few minutes.  No leak.  I thought maybe running it created a vaccuum in the tank to help seal the gasket. I turned the Speedster off.

I removed the gas cap carefully to remove any pressure.  Then, with paper towels near by, I carefully loosened a gasket screw.  Dry.  Another screw.  Dry.

Suffice to say the gas level was now low enough.  I still plan to replace the gasket...

Thanks again,

Looks like you're halfway there -  Pulled the screws and no leaks!   👍

Give us an update when you've swapped the gasket.  BTW:  The sender only lines up with the screw holes one way (one screw is slightly moved) so you can't screw up the direction of the float.

Great Persistence!!