The V12 is a very strange set up. Yes, 17inHG is sucking on the PCV valve, which limits that vaccum (let’s say by 50%?) which then sources from both the crankcase AND the B airbox. So no crankcase ventilation, and no negative pressure on crankcase. It’s basically sucking a bit of the oil vapors out at vacuum, and allowing a relief (B air box) during positive pressure/0 vacuum (better than your seals, I guess!)
During positive crankcase pressure, such as flooring it, now any blowby crankcase pressure from the pistons goes out the pig snout and into the PCV valve, where some goes into intake manifolds via the y tubes, and some goes into B airbox. So instead of sucking out the oil vapors, they are now being forced out. Like I said earlier, better here than your seals.
We still aren’t certain what your setup is. If you have the OEM setup with a catch can simply between the pigsnout and the Y tubes, and have eliminated the pcv valve, like Dave said, that’s wrong. If it’s between PCV valve and Y tubes, I guess that should be working?
My setup in a nutshell (very similar to Dave’s) - I stuck my PCV valve INTO the pig snout. I then ran a hose from it to a catch can, and then that catch can ran a hose to the Y tubes. I then used the front timing cover plug as an external air supply, which gets filtered air from the A airbox. I get from 0 to 1inHG at dipstick at idle. I’m not too concerned with maintaining a negative crankcase pressure at idle, but I like that the crankcase is well ventilated now like they used to do on old V8s. My guess is that while cruising, where you can get 20inHg vacuum, i am hopefully getting some negative crankcase pressure. I get pretty much no oil in throttle bodies, and of course 0 oil in B airbox now that I’ve eliminated that section. I’m sure the engine is happier too. I’ve tested the A airbox supply hose, and see no oil in that. So during positive crankcase pressure, I have two large exits for all of that pressure to go. The catch can plumbing I think serves as a great relief, due to it’s volume. And hopefully my rings are not allowing too much to get past (Last I checked, I was at 210s/220s cylinder compression on a warm engine) That’s where I think using synthetic oil has helped those rings get cleaned and seal better.
On a side note, the oem setup is mediocre, but ok. But i found out, make sure to use Jaguar PCV valve! A few years ago, I went with cheap $5 valve as suggested in the book. Started getting some oil leaks at various seals. Discovered that the Jag valve allows much more volume than standard pcv valves (a good 50-100 rpm at idle). With the strange oem setup, i guess you need a bit more vacuum pulling to get any pull on the crankcase.