The 10-4 torr*L/sec leak on BSC5's dome outer O-ring has been located and marked with white tape for future reference -> This leak will have to be addressed at some point in order to pump this volume with the nominal ion pump, i.e., BSC5's dome will need to come off -> This looks to be the only appreciable leak for this annulus volume -> Also, the venting of this annulus space did not affect the 1 x 10-7 torr pressure indicated on PT510B. 

I improved the 2 x 10-6 torr*L/sec leak of the IP12 gate valve bonnet via - "wait for it!" - TIGHTENING the bonnet screws (it's what I do - required turning down the o.d. of a 13mm socket to a very thin wall thickness) to a more tolerable 1.5 x 10-9 torr*L/sec.  Some of these screws were found to be very loose - others, not so much.  This make/model of valve used to isolate the 2500 L/sec ion pumps have noticeable as-built QC issues.  Among these are the use of hex-head cap screws for the bonnet joint instead of 12-pt cap screws-resulting in damning interferences which prevent the use of unmodified tools.  I suspect that the as-found loose screws were loose due to these tool interferences.  I'll scrutinize these joints (site-wide) as opportunities present themselves - chances are others are leaking for the same reason.