I also had an 850 with a dipstick pressure problem. Your post made me curious and I examined my flame trap yesterday and found the narrow and short vacuum which connects to the flame trap holder was disconnected. I removed the 6 inch plastic hose with rubber connectors from the vacuum tower connection also. Out of curiosity, I probed the hose and found it to blocked solid with carbonized (from the engine heat no doubt) oil. Four of the 6 inches was blocked as if concrete had been poured in it. I could not free the line so I bought a new length of hose and two rubber connectors. The cost of parts was $2. The labor time to disassemble and reinstall was 30 minutes. I took the car out for a ride at high speeds and stopped with engine on and pulled out the dipstick. There are no signs of excess pressure any more!

You might consider trying this. It is cheaper than the rear main seal and oil trap I had installed in my 96 850 before I was able to identify the real cause of my car problems.