Brake master possible leak

Photos would sure help, or poke around in Google Images to find the correct terms for parts.

If "diaphragm" means the "vacuum booster", there is no seal there. Indeed, there is a gap in the plastic at the bottom to equalize pressure or perhaps drain leaking fluid. But in A-bodies the booster doesn't bolt direct to the firewall, so ditto the "huh".

If you mean a manual brake car and the thin white foam between it and the firewall, that is just a dust seal and I recall referred to as "paint seal" or such. You can buy a set of foam from places like DMC, or just cut out yourself (like I do) from the white foam bags in retail electronics packaging.