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I was told :
When the seals go bad on the primary piston (the shiny silver inside the snout), fluid will leak out and collect between the master and the vacuum booster. The little drain hole is there to *try* and keep the fluid from building up in the gap and being forced (or sucked) past the booster's seals, slowly filling the internal chambers with fluid and eliminating the "boost" part of "vacuum booster".
Time for a new master, and as suggested, check the booster for fluid inside, or any air leaking in/out around the booster's front pushrod seal.
SO:
I pulled it off and the pushrod came out with it - It appears to have a small spline on the front that was stuck a bit with age. There is no damage to the seal, it is still supple and there is no fluid behind it. It does not appear to be a pressure fit or anything - just a seal held in place by the bolting together of the two pieces.
I experienced no symptoms that I had read elsewhere of a damaged or malfunctioning booster, so I think I can assume all is ok on that front.
My parts guy here has suggested a Cardone 10-1423 as a replacement. 1976 was a transition year - I ran into that issue on another repair I had to do previously - mine has no bleeder screws on it, but I am not sure if that Cardone unit has the drain hole. It certainly seems like a good idea on the basis of this experience. I'll check the Raybestos MC36367 unit out further.
Time for research on the e-brake, etc. Thank you very much for the expert advice!