oil blow by

I have an oil leak that's driving me crazy on my '87 MM with the 2.6. Ever since I switched to the Weber 3236 last November, I have a persistent oil leak coming from the bottom of the bell housing. I won't leak at idle, no matter how long it sits. After about a 15 minute drive, when I stop it drops about a 4" diameter area for about half an hour. I just changed the tranny a week ago and the one year old rear main seal is good. The PCV valve appears to be good. When I take the oil filler cap off at idle, there is pretty good air pressure coming out. The hose at the back of the valve cover that connects to the air cleaner has no pressure or air flow when I disconnect it-shouldn't there be some? Could this be the problem? Crankcase pressure not being relieved through this breather hose, causing oil blow by getting past my rear main seal or am I totally off base here? I'm going to pull the valve cover to check the filter thing under that hose to see why I'm not getting any flow there, but I welcome any input from better minds than mine! ( which is pretty much anyone that can blink and walk at the same time.
Thanks

could be the plug on the back of the head or the valve cover gasket running down the back of the block. Also could be the head gasket leaking oil, but that would be really rare

thanks pennyman. I'm going to pull the cover saturday and scope it out.

Steam clean it, isolate the leak.