the rear breather is for both crank and cam cover ventilation 'off idle' intake pipe via pcv is breather for both 'on idle' there is no pressure in the rear pipe whatso ever unless (sure you mentioned having the earlier pipe) you hae the check valve pipe and it's faulty - in which case remove the check valve
only way it would leak is if the rear pipe check valve failed and it's creating pressure off idle, i'd expect exhaust gas to smell a little oilier than usual too.it's worth a check but sounds more gasket / mating surface related. was it an OEM gasket? and you fitted bolts through gasket before cover installation?