I suspect that the ventilation hose connected at the airbox experiences a very slight vacuum, which is created by the pressure drop across the air filter as the engine breathes. But now, yours not only doesn't have that slight vacuum, it instead may have a slight positive pressure. This slight pressure can occur from the restriction caused by the extra filter -- the pressure increase will depend on the flow rate.

The "PCV," as you call it (it's really called the "flame trap"), has a small vacuum line, about 1/4" OD, that connects to the top of the "Y" fitting. The other end attaches to a small brass nipple on the intake manifold. That hose and the fitting are notorious for plugging with carbonized varnish, blocking the vacuum.

Be absolutely sure that the oil spearator (aka "breather box") and the flame arrestor (inside the flame trap) are clear. If either plugs up (not uncommon, particularly on a car that had neglected service) then your crankcase will develop enough positive pressure to blow oil past all the seals, and even pop a seal outta its housing (BTDT).
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Don Foster (near Cape Cod, MA)