Gas pouring out of gas tank...

Hello.. need help to figure out WHY gas is pouring out of gas tank...
Car is a rod.. 1936 Chrysler, has a 318 engine with a mid 80's auxiliary gas tank. mounted under truck.
All OK for first 20 plus years.. Last 2 years gas on occasion pushes itself up fuller spout and out , screwing up fender pain. Even after 100 KM since last fill up.
Car just sitting in car port... for days before it does this..
siphoned out a liter or so and happens again without cap on

Please and Thank You
We do appreciate you taking your time to help us figure this one out..
as it is a mystery to us!

Thanks again
Arev

Sounds like the tank is pressurizing. I'm not sure of your set up, but if you installed a vented tank then your vent may be plugged. If you have a vented cap, then you may need to replace the cap.

Thank You for your reply..
the strange thing is . it does it without the cap on!
and we did replace the cap.. and also had it tested and it works find..
this is a head scratcher for sure..
the guy that build the car had it for 25 years before we bought it.. and we had it for 10 years.. and just the last 2 years this is happening.
there is no vent on the gas tank other than the gas cap..
you have any other thoughts?

Thanks again