To have a "closed loop" system, inlet air needs to come from the clean side of the filter. Some modern air cleaners have a small horsehair style filter in the side of the housing for crankcase inlet air so a manufacturer can technically call the system "closed". Some people cut the road draft tube off and weld a washer in its place for a grommet that holds the PCV valve. OEM standpipes put the PCV up around valve cover height to keep liquid oil from being drawn out of the crankcase.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!