Why does a PCV need a breather at all?

[davepl]

Why don't we just run full manifold vacuum to the crankcase -without- a PCV, or even without a breather, so that it builds up a full 20 inches of vacuum in the crankcase?

Is the fresh air system an emissions thing? What does that accomplish?

As I have my PCV on my LS motor right now it's capped off, so I get massive vacuum in the crankcase. That's not "correct", I need to open one bleed port to the atmosphere.... but why?

My best guess is that even under vacuum, without a fresh air path, you've still got blowby gasses that lead to sludge and acidity, just at a lower level because it's under vacuum. With a fresh air intake, however, that's always replaced by fresh air. But that's totally a random guess.