The effect of a catch can on oil leaks

[the evil twin]

PCV shouldn't be vented to atmosphere, that defeats the whole purpose.

The whole concept of PCV is that the gas blow by from the cylinders into the crankcase is 'positive' vented back thru the induction to be burnt and not dumped direct to atmosphere.

There should be a PCV valve in the vent line from the rocker back to the induction point and the Catch Can has the job of capturing the oil condensate so it doesn't reach the induction and foul the engine intake.
When you fit the Catch Can you still need the downstream flow after the Can to go to induction not atmosphere