Jens,
I realise that you don't know me, but I'm with you on the vacuum thing.
The singles run under considerable vacuum. I temporarily replaced the breather valve on a single with a 3mm sheet of soft silicone rubber.( The original was off for a clean up.)You wouldn't believe the deformation in that rubber! I would imagine that dust etc was not thought to be a problem, or couldn't be dealt with using 1920's technology.
The twins are a whole different ball game.They are "designed" to run under pressure. The reason I say this is that there is no mechanism for generating vacuum, but there will always be some blowby (however slight). Ergo positive pressure.
My 'roid twin used to leak oil from the end of the camshaft, mains, tappets (slight, and probably not a bad thing) and anything with the slightest clearance. I also have a Dursley twin and I fully expect it to leak volumes from the sliding oil filler cover.(I no longer use the 'roid, and the genuine article is a "long term project"
at least 2 years so far, and counting....)
Many moons ago,I suggested putting a
slight restriction in the intake to generate some vacuum for the breather (never tried it, got mains power!). There was talk on this forum that putting extra air into a diesel is counter - productive if you don't need the extra power. Having
slightly less air shouldn't be a problem if you only want 80% or so power. My Powerline twin came fitted with a restrictor in the form of an undersized hole in a gasket on the inlet manifold!
You are already part way there, try a larger hose and some restriction. If you don't like it take it off!
Hell, tinkering is what it's all about isn't it?
cheers,
brian