94 850 smoking, revving, hissing

[Matiss]

I did take the oil filler cap off- good question though, because I thought maybe it was to see if the seal on that was faulty too.
The pcv hose on top is nasty and cracked- going to replace that, as there is definitely plenty of oil collected on top of the head (either from that or the cap, or both). The smoke may be coming only from that, but why would it smoke only or more at idle (or is it just that I may not see it while driving??).

Quick update- underneath the air filter box there is a vacuum hose that goes from the "tree" by the fuel rail and connects to the wall/body at the back of the engine compartment. It is connected to the brake booster, because the stiff/hard brake pedal issue (which I now realize I neglected in the original post) is resolved.
The car starts normally, runs as smooth as it ever has, does not rev at idle, and up til now, has yet to smoke. However, the random/idling stalling continues.

Any ideas on the stalling issue? Sorry for leaving out the critical brake booster issue, and thanks for the help!

[tryingbe]

Vacuum leak would cause stalling.