Willys M38 tank volume only half

Hello,

I have a question for my Willys M38.

The gas tank capacity should be 10-1/2 gallons (38liters).

Unfortunately I only can fill in the half (approx. 20 liters).
After that the gas tank is full. The gas tank is a original remade version from a CJ3B and about 10 years old. The issue was directly after installing. I ignored it, but now i want to drive longer distance and need to fix this issue.

Someone know this issue?

Is this maybe a ventilation issue ?
Or it will help if someone can explain how the gas tank is build up inside (is there a partition wall inside) ?


Body: Willys M38
Undercarriage: Willys CJ3B
Engine: MB Diesel

Thank you for help