Well, I've been out of town for a bit, and way behind on answering questions. I apologize.
Because I couldn't figure out (at the time) an easy way to check out/isolate the booster operation without pulling it out, I swapped out the booster for an '84 unit that I had planned on eventually installing (to take several pounds off the car). I haven't driven the car yet, but with the engine running (and hence vacuum to the booster) the pedal does feel reasonably firm, with just a little additional pedal movement compared to the movement prior to starting the engine. This agrees with what sky65 said, and what I experienced often through the years with this car. I'm hoping to be able to take the car out for a test drive tomorrow to see what the brakes feel like when the car is actually moving (although there shouldn't be a great deal of difference).
I'll confess my lack of total understanding of the workings of the booster. I (believe I) mentioned earlier that the internal reaction disc was missing, possibly from when I originally swapped out the original m/c for the present C5 m/c. I had to remove and shorten the m/c pushrod to work with this m/c, and perhaps it was then that I lost the disc. I just don't ever remember seeing that part. Did that cause a long term problem? I don't know.
And obviously, the '69 booster is 46 years old. I don't know all the failure modes of a booster. In hindsight I'm curious if removing the m/c and bolting a dummy resistance to the front of the booster, and then monitoring the pedal travel with and without vacuum assist would have given me some useful information.
The "new" '84 booster is a single diaphragm unit. Hopefully that makes it easier to diagnose any future problems.
That's where I'm at, at the moment.
