Wow Paul, thanks for the in-depth go-over!
The 12-14 bar spring force is weird. Are you using line pressure or purely boiler pressure generated by steam? That bit wasn't clear to me. I have a theory myself after trying both boiler-preinfused and line-pressure pre-infused levers. You need a higher spring rating for boiler-preinfusion (0.8-1.2 bar) compared to line-fed (2-4 bar). The reason is the higher initial pre-infusion removes more air from the group and the lever kicks in earlier (because less air to compress). If your boiler is pressurized by line pressure, it could be the reason. Boiler pressure= use a high spring rating ; Line pressure = use a low spring rating. I've measured a recent Bosco-style group (on a Profitec 800). It's about 9-10 bar with flow (it needs a good pull strength so I believe it's double spring).
As for the E61 screen sitting too low, it can be made to work but not easy (for us - probably easy pie for ya
). Rick (Coffee_machinist) has trimmed some material from the sleeve so the screen can sit higher - best of both world.
2nd post pic no. 8.http://coffeesnobs.com.au/brewing-equip ... ution.html
After seeing your photos, I have to say I am glad to have an older style lever grouphead with pop-on screen.
Never seen that much build up on mine! And completely agree with you that levers should be easy and no thoughts required.
p/s: I love your Cafelat silicone grouphead seal and lucky me, it fits my Brugnetti Aurora with a bit of help. Can't wait to try your silicone piston seal IF(big if) that's in the pipeline. It uses Faema's W and V seals.