O-ring seals are good by design, but they have to have smooth surfaces to seal against and a little compaction as they come together, they don't rely on screw pressure. Take the o-ring off and check carefully with magnifier glasses or something similar and use your finger as well to see/feel imperfections in the groove channel where the o-ring sits and the inner surface of the vessel. The groove surfaces have to be smooth, as well as the surface of the vessel that the outer o-ring seals against. I have a brand new 55L Fermzilla, as yet unused and untested for pressure. After reading these posts, I checked mine, and found a very slight manufacturing moulding join bump (x2) on the inner ring of the o-ring seat of the lid (under the o-ring). This sort of attention to detail is important! I carefully and very lightly filed the moulding join bumps flat so that the o-ring can sit flat against the surface and do it's job properly. As yet still untested, but from previous experience dealing with o-ring seals, this usually fixes the problem. I'll borrow a line from the Lion King movie....LOOK HARDER!!!!