Please try it and report back. Try to take some vacuum measurements at idle and under boost. Just be aware, that when you have a check valve, that does protect from a backfire but at the same time, that completely halts airflow toward your crank case. This means at idle, you may have the 'howling seals' issue because you've completely restricted this air flow. The flapper is a swing valve has a hole in the center of the 'flap' so it behaves as an orifice restriction when closed, it doesn't seal completely like your check valve will.
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So what you'd need, technically, is another check valve that has a cracking pressure of a few psi, something that engine vacuum can overcome. Then put a filter on the other side since air will be drawn in.
I'm not sure if I'll keep the factory flapper or not. I may just pull the flapper, put the stone filter back in my high side catch can and let that stone filter do the 'restricting'. This does add a blockage when it comes to on-boost situations and increases my potential to have positive crank case pressure, but if that occurs I'll address that when I notice a problem with turbo oil drainage, smoking, etc. But I'm not putting out any crazy power numbers so my suspicion is that it will be okay. But currently the flapper is still there, just relocated behind my high side catch can.
My setup is explained in detail here:
https://bmw.spoolstreet.com/threads/2009-e93-ad-e-single-turbo-build.959/page-13#post-52417
6AN isn't too small, because the PCV side, aka low side only operates in one direction, and it operates under vacuum only, its not a Vent To Atmosphere line which needs to be free of pressure obstructions.
From what I can tell, the RB external valve is simply a non-spring loaded check valve. The cracking pressure is pretty much nil, you can easily switch it back and forth using your mouth with ease.
Personally, I run a Russel check valve between my intake manifold and my low side catch can so my catch can isn't getting pressurized. You can imagine what Kind of mess would occur if a line came loose or something broke. This means I could actually just eliminate the RB PCV valve and just keep this secondary check valve since they are accomplishing the same thing. But Russell isn't exactly Toyota, and if it failed the last thing I would want is a bunch of boost spraying oil from my catch can back into my motor. Not to mention, the Russell valve has a higher cracking pressure, I'm actually looking for a 6AN valve that is not.
The problem with ball check valves, is that they can slam closed when pressure hits from one direction, such as from boost onset and this can crack them. But last I heard, RB is using a Toyoto part, and OEM parts are pretty dang reliable generally speaking.
The RB valve allows the engine vacuum when at idle or deceleration to suck on the crank case, into the intake manifold. But when you step on the gas, the boost that enters the manifold tries to go back up that same line, but the RB valve is there and closes, preventing your boost pressure from entering the crank case. Meanwhile, blowby from the cylinders is exiting the flapper valve into the rear inlet, intake cone/pipe, VTA, through 10AN flapper delete or whatever you happen to have there. This is a larger line beacuse there is no vacuum source on the other side to suck it out(besides the previously mentioned intake pipe/cone options which have very limited vacuum). You hope this is happening without building up positive pressure as this causes the oil drains on the turbos to get backed up, causing smoking problems and other issues. This is why they use the BMS catch can, because it has virtually no filtration element and poses little restriction to the blowby fumes trying to escape the motor.