Back when my engine was completely disassembled, as a mockup, I placed the timing gears on a table with the breather in place and the position of the breather port marked on the table. That's been a while and I should should have kept better records, but, as I recall, the hole in the rotating breather passes the breather port at 90 degrees AFTDC. By this, I have assumed that the breather is not intented to change the pressure in the crankcase. My take on it is that the breather is only intended to keep the crakcase somewhere around normal atmospheric pressure so that oil can be pumped in and sucked out. If you open a full-time vent, it is always pulsating, to what negative affect, I wouldn't know. However, I assume Mr. Hopwood had an opinion on it or BSA would not have gone to such lengths (no pun intended) to duct the breather out behind the timing cover.
Looking forward to the fifteen corrections to my theory, upcoming.
Richard L.