I have to admit, I don't understand how TTJohn's problem correlates with Drew's problem. Regarding pressure in the case, in looking at the timed breather (some years ago), it appeard to me that breathing occured 90 degrees ATDC. It also seemed to me that it's main purpose was to have a fairly neutral pressure in the crankcase so oil could be pumped in and sucked out. Regarless, I believe some pressure or vacuum inside the crankcase is normal.
Drew, your timing cover appears to be severely cracked and I'm wondering if it's not due to someone in the past trying to screw down the non-tach cover while a tach driving bolt was in place on the mag pinion. This could easily lead to a cracked and distorted cover that not only leaks from the crack but also from the gasket joint.
Richard L.
Edit1: On another look, maybe the crack occurred due to the impact that caused the deep scuff near the top of the cover.
Edit2: Yeah, yeah, yeah, I missed the point that TTJohn's bad rings and pistons meant a lot of blow-by and excess crankcase pressure. Also, I understand how pistons working against pressure is not good for power, but if oil can't get in, pistons won't be working at all.
Edit3: Hey, I just realized, plunger breather operation is different from swingarm, isn't it?