With fueltank installed it was time for leak-test. I filled some fuel and applied air pressure. At 0,1 bar tank starts balloning, so I kept this pressure. No fuel was leaking from lower part, and I used soap-water outside fuel level indicator, returnline and ventilation hose. No fault found. I found the filler cap has a one-way vale inside, and together with a tiny ventilation hose it must be safe for vacuum problems.
