I just finished my tanks. Here is how I did it. I bought some 1/4" plactic tubing from home dpeot. Heated the end upmand pushed it over the tank's vent to flare it. Added a B nut and AN sleeve and hand tightened it onto the vent fitting. Blew into the tank until i couldnt blow any more, took a swige of water and blew that into the tube for about 30 inches then kept on blowing until it filled up the manometer half way ( both sides level) . Stoped blowing and the manometer settled at about 24 " of water, which is just shy of 1 psi. Then i bubble tested all the joints and let it sit over night. My breath in the tank cooled over night and I lost 1.5" of water, but the bubble test showed no leakage.
After reworking the second tank, i add a second tube on the fuel pickup. One with water for the manometer, one to pressurize. This made it easier to fill with my breath without have to push a swig of water into the tube. I just used a 1/4 inch punch to seal up the pressurization line. Easy peasey.
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Extra dues paid 2017, worth every penny
RV9A- Status: tail feathers done less tips