I've got an unused .4 head Assembly, will change the seals and o-rings from this to the .2 to see if there is any issue with them!!It is also possible that the ring is injured. I had the problem already and it was a new part. Cleaning with solvents can also make the rubber porous. The needle cap should be clean before the ring is pushed over the thread.
Almost four years, i bought it to paint my plastic models, it was the first time that i had an issue, and as Micha said it was a minor issue and easily solved, with his knowledge of course!! I never had thought that a new o-ring could be the problem!!I'm curious- how long have you had this brush? A lot of people seem to have trouble with the Evos...