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You can verify an excess of pressure by loosening the oil filler cap and seeing if it will rest on the engine while it is running. If the cap wants to lift up much at all, there is positive pressure which should not be there.
I think you can diagnose the oil trap in place. There are 4 places the pressure can go (assuming you don't have any blown oil seals). 1) Dipstick, 2) Vapor tube that runs from the fitting on the cylinder head (under the sparkplug wire cover) to the oil trap 3) Vapor tube that runs from the oil separator to the flame trap 4) Passage through the oil trap to the crankcase.
I would remove the tube from the cylinder head and plug the end of it. Also remove the tube at the flame trap. Blow a little air thru the tube at the flame trap end. If the path through separator to crankcase is not clogged, this air should escape out via the dipstick, oil filler cap, or the fitting on the cylinder head. Conversely, if you close the oil filler cap, the fitting on the head, and the dipstick (blocking all exit via the crankcase), air should be able to go out through the tube which had been on the cylinder head.
I once had trouble with overpressure that remained after cleaning the flame trap well. It turned out the tube from the oil sep to the flame trap was blocked with crud (sitting an inch or 2 back from the flame trap end). Also as others have pointed out, the small line from the flame trap housing that goes direct to full manifold vacuum can clog badly.
I guess these tubings can get quite brittle over time so they could crack when handled.
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