given the mileage, i'd say the oil seperator is clogged. at $40, it's cheap insurance. i like to change them at 100k or so. good luck, chuck.

If you're sure the flame trap is clean, the next suspect would be the oil separator box. To test this, you'll need to take the flame trap off, get a length of 5/8" vinyl tubing, and hook it up to where the flame trap hose comes off of the oil separator. Take the oil cap off and blow through the vinyl hose. The air should not be obstructed or have resistance, and you'll feel it coming through the oil filler hole. If there's any resistance to the air, take off the oil separator box to clean it, or replace it.

You should definitely have negative crankcase pressure.

Yes, there should be negative pressure. The flame trap crankcase ventilation system is supposed to scavenge out any vapors in the crankcase and burn them. In olden times these vapors were just let out into the environment.

Positive pressure in the crankcase will cause leaks at the filler and seals. Did you check the brass orifice where the small flame trap hose connects to the intake manifold? The opening inside this is pretty small and easily plugged. Also be sure the larger flame trap hose is tight and not split at each end where it connects.

Is the flame trap itself open? These can plug up. Pull off the flame trap, slip a 5/8 OD hose on the flame trap, open the oil filler cap, and try blowing through the 5/8 hose. You should be able to freely blow through it.

If all if as it should be with the above, you may have excessive blow-by around the pistons. This could be caused by worn or stuck rings in the pistons. I've had fairly good results in freeing stuck rings by using a slow acting engine cleaner. The one I've used is Rislone. Use a quart of Rislone in place of a quart of oil at an oil change. Drive for 1-3000 miles and change oil and filter. The Rislone is a slow acting cleaner which won't suddenly release lots of sludge and other debris which might plug up the oil pump screen or oil passages. There are other cleaners but I'm not familiar with them. For a huge amount of discussion, opinions, etc. on this see the following URL:

http://theoildrop.server101.com/ubb/ultimatebb.php

If you leave my car's filler cap untwisted, them push down on it
briefly to make the seal grip, the cap will suck down tight at
idle. It's a plastic cap like yours, and it will jump around
until pushed down briefly because it's not heavy enough to force
the seal to grip.
.
-b.

I can't say with exactness, but under moderate-to-high manifold vacuum conditions, I believe there should be negative pressure under the filler cap. It will be slight but should be noticeable.