Fuel tanks

you beat me to it, yes, very low pressure.
Another way to find the leak is to use put a gallon of avgas in the tank, spray Dye check developer, which is a fine white powder in a water carrier, where you think the leak is. you can rotate the tank to get fuel over all the seams if you are not sure where it is leaking. The dye in the avgas will make the leak really stand out.

I have done this to check my tanks for leaks (and to push out a dent where someone had stepped on the wing tank), and it works great. But a word of caution, don't over pressurize. Mouth pressure is plenty. I used a toy balloon to maintain the pressure while checking for leaks.

Once you find the leaking area, depending on where it is, you could use the JB-Weld (or Marine-Tex, white vs. grey), Pro-Seal, or a pre-fab patch like www.clickbond.com/products/click-patch

Ok, even easier and simpler, get spray bottle with water and a few teaspoons of dish soap, pressurize tank lightly, where you see the bubbles when you spray the seams is where your leak is. Dry site and apply JB-Weld over leak. 34 years chasing leaks on LNG tankers you learn dome easy tricks, a SNOOP TEST

Talk to some RV builders , they build wet wings and use a material to seal their tanks. If you can pull the tank that makes it easier to spin and rotate the tank and cover all the interior surfaces.
Aircraft Spruce and Specialty sells a product to seal old tanks.
www.aircraftspruce.com/catalog/appages/g...ntent=All%20Products