After some hundreds of Kms and 6cm of crankcase vacuum at idle and 3.000tr/min,
I still find half a litre or oil in the air filter housing.

So what's wrong with it?

Taskos,
I doubt that there's anything wrong with your breather, which is doing its job of venting crankcase fumes into the air filter box as well as creating a vacuum inside the crankcase.

The problem is most probably with your engine, specifically excessive 'blowby' past the piston rings, so that the breather is unable to condense all of the oil from the vapour which passes through the breather.

A leakdown test would be most appropriate method of checking the condition of the pistons and rings.
http://en.wikipedia.org/wiki/Leak-down_tester

Also
http://www.youtube.com/watch?v=7UcOn8OEt0Y ( Making a homemade tester.)
&
http://www.youtube.com/watch?v=GvG22YMOzUI ( Using the tester.)

ken

I had the same problem in my turbo car, I solved it with an oilcatchtank, I've put the oilcatch tank between renyflard and airfilter, so the oil wouldn't reach the oilfilter, ofcourse this isn't the proper solution (see Ken's story) but when this is really good engine apart from this, this is a good way to go..... You can drain it le'ts say every 1000km or so and that's it....

you can make it yourself it is just a bit of pipefitting, otherwise take a look at this..

http://www.ebay.com/itm/HIGH-QUALITY-CAPACITY-BILLET-ALUMINUM-OIL-CATCH-RESERVOIR-TANK-CRANKCASE-BLUE-/400611608883?pt=Motors_Car_Truck_Parts_Accessories&hash=item5d46500933&vxp=mtr

pistons/bores are worn, causing blowbye
get a rebore and 2 new pistons

Just a quick comment [tip?] .....a couple [?] of years ago I destroyed * one cylinder of my engine .....the "blowby" was so bad that the carb choked up on oil fumes .....I got home [on 301cc's] by removing the air cleaner to carb. hose [it was the only way the engine was gonna run].

* Broken rings,scored barrel,cracked head,broken piston etc .... the engine had overheated,Ken rebuilt it for me [btw Ken ....its still going like a good 'un ... ]

T.

Ive nearly always found the same - this tears race car had the same syptoms and after much it could be and it cant be took the top end off honed the barrels and fitted new rings. 24hrs later 1tablespoon of oil in the catch tank it had been dumping half a litre in abour an hour

Viking , did you ever get to the bottom of your oil dump issue?

Haha you knew I would read this. No haven't had time to strip the engine since returning from Spain. I'm off to Denmark for Christmas, so maybe next year sometimes

On a positive note.
I'm feeding the chassis with the oil reducing the risk of rust worm attack. Not really sure if I want to fix it. Engine is running well so no worries

6cm of vacuum is not a lot - on mine I have 40cm at idle

That's a real big oil catch tank then

And at full chat/full load, which is when a 'pressurised' crankcase is going to make itself known?
Similar scenario to that of ignition timing; what's happening at tickover isn't of much importance, it's what's happening when the engine is really working for its living which counts.

I once fitted the manometer inside a car, so that it could be monitored by the passenger as the car was driven.
Anyone else tried this?

ken