I noticed that it was a bit deficient when I had part # 6 out. It pretty much just seals air, it is something to consider I suppose.I was wondering how you are getting on with your slight leakage problem? If it helps after totally stripping down two of my Tobeco's and replacing all o rings i found on both the 5 was the wrong size.It was smaller,should be 7 *1.5 and it was 5*1.5.
It didn't have much "drag" on assembly.
Edit: Every time that I change #10 & 11 the leaking (minor) stops for a couple of weeks.