Because the fluid level in the rear reservoir very slowly (like a half ounce or so every six weeks) goes down, w/o any fluid appearing outside the vehicle, and no brake grabbing issues such as are common with leaking slave cylinders. Also, I get s sucking noise on pedal application intermittently, so there is something going on vacuum-y.

Both mechanics I work with independently suggested the booster-ingesting-fluid idea. Both are old car guys ('69 Dart, '72 Mach One) at a garage that specializes in old/foreign/race (and all of the above in combined permutations) cars.

My current plan is to wait until the problem becomes much worse, so that when we take things apart it will much more likely be obvious what's wrong - trying to track down a 1oz / 3 moths leak is a PITA. Was really just looking for sources/availability in case the booster needs replacing at that time; it's old and sometimes you have to replace old stuff. I know pretty much everything else in the brake system is easy to get.

In any case, looks like the booster is probably unavailable - I will look into finding a re-builder. I can always ship it out to Booster Dewey if I can't find anyone closer, if it winds up needs doing.
-Kle.