I bought the .20 Evanix Blizzard S10 last year in July. Since then it had a leak that was audible and it lost about 10 bar overnight.
Since then I searched for the correct O-rings and tried all different kinds. I even got a complete o-ring kit from Louis, but the same leak manifested time and again.
Upon examining the action there are two small (0.1-0.3mm) indentations machined. If the valve housing are screwed in the action then it (technically) doesn't seal perfectly and will leak at the two opposite sides.
My remedy for this problem was to make some sort of "gasket" or "membrane" from surgical gloves where a a cutout was made roughly the same diameter as the cylinder, with a smaller hole where the valve pin and other o-rings will fit through. This membrane was smeared with silicon grease (From the Hill company) on both sides. The valve housing (with the cylinder) was screwed into the action to ensure a tight seal (by means of the "membrane") between these two metal parts .
So far no leaks are noticeable.