I don't have an answer for your problem, but I can offer some insight about your current standings. My '67 with a 468ci motor produces about 14" of vacuum and the power brakes work flawlessly...but at first were terrible (mucho pedal pressure)...turned out I just had a vacuum leak that only reared it's ugly head when brakes were applied, which was a PITA to track down. Probably not your issue, but with you having 15" of vacuum, that should be plenty...
Good luck, somebody on here will have the answer, fear not.
