Update...
Today, FRUSTRATION...
I received the JBM carb holders and diaphragms (along with a new JIS BS38 carb screw set...none of which fit the top cover so I can replace the screw I stripped....but I digress). Anyway, I installed the carb holders without any problem and brought the carbs home with me to perform the diaphragm-ectomies (too dark to work at the storage place where I keep the bike), so I haven't yet test-run the engine to see if they eliminated the air leaks and/or improved the engine's performance.
However, there really isn't anything wrong with the old manifolds (carb holders) that I can see. Although I didn't photograph it, I tried stretching the material in search of cracks, but found none. They look nearly new, with pliable and solid rubber material (i.e., no cracks/damage). So, unless my recent and repeated r&r'ing of the carbs ovaled or widened the opening sufficiently to make it impossible for the clamps to create a seal around the carb openings, I'm not seeing much difference between the new and "old" sets of manifolds (carb holders).
As for the carbs, I remain mightily confused. Now, with the JBM diaphragms installed, I'm seeing the same functionality I saw with the other "new" piston/diaphragms that I just installed last week. The right carb--the side that was causing all the problems--seems to maintain a good, proper vacuum...if I raise the piston and block the curved port atop the intake side of the carb, the piston takes ~15 seconds to gently glide back down. Normal. However, the left carb--the side that IS running well--seems to have a questionable vacuum...if I repeat the piston-drop test, the piston drops in just a few seconds. Much quicker than the other carb. But, again, the side with the fast-dropping piston's the side that IS running well.
So, at this point, I don't know which way's up....I mean the good carb's got a poor vacuum and the bad carb's got good (normal) vacuum and both've been rebuilt multiple times with Mikuni parts (including new Mikuni needles) and JBM diaphragms. What's left? Ugh..
So, I guess today's question is: Should I be concerned with the differing piston drop rates between the two carbs--regardless of whether it's related to the air leaks--or just put everything back together, as-is, and see how the engine runs with the new manifolds? Or, if the fast-dropping piston in the one carb IS an issue....having replaced the diaphragms with JBM's, what else should I check that affects the piston-drop speed/vacuum in that upper chamber?