...In theory, it should still be placing the crankcase under a slight vacuum...
Correct, and that is what is happening. Blowby has nowhere else to go (no draft tube, sealed valve cover) and is routed through top valve cover hose and PCV valve to the intake manifold then burned in the internal combustion process. Meanwhile, fresh air is introduced through the air cleaner and rear valve cover hose.
Can feel a definite vacuum pull when front hose is removed from valve cover while engine is running.
