I am having a large problem with my '92 940 turbo. I seem to have massive oil leaks... mostly out of the oil pan gasket but in multiple places. This makes me think that the crankcase breather is plugged... so I cleaned the box and checked the lines etc. If I blow in the breather line, it doesn't feel blocked... it just seems like I'm blowing into a sealed box. With the engine running, I can feel pulsations at the breather line. The oil cap will not stay on the valve cover with the engine running.
I did a test which I call the "bag test", and placed a plastic bag over the oil cap hole with the engine running.(it should suck down for vacuum and blow up for pressure) It vibrates up and down like a reed (with associated noise).
Are pressure pulsations in the crankcase like this normal? I'm wondering whether I just have excessive blowby... but I don't know what would have caused this. I could change the gaskets... but I think that might be the bandage and not the fix. If anyone has any suggestions I would be very grateful. Thanks!

James Peverill