I took the booster off my 1984 (or 1985, forget). I recall only a foam gasket "dust seal" between booster and firewall, as on most cars. As o.p. said, there is an O-ring between MC and booster, which seals vacuum. That seems common on boosters which are thin, pancake type, like the Girling in my 300D's. In my 1960's Chryslers, there is an internal rubber bellow around the booster output rod, so no need for a vacuum seal at the MC. Indeed there is an opening to let any brake fluid which leaks past the piston drip out (instead of go into booster as in 300D).

Many boosters have an adjustable tip on the output rod. You adjust is so it just barely pushes the piston in when bolting up. I don't recall if my M-B Girling booster had that. Many people seem to think the brake pedal should bottom out against the brake switch when released. The switch should only sense the brake pedal and not prevent it from moving where it wants to rest. Insure your switch isn't limiting pedal motion. If so, it might be like "riding the brakes", which would actuate the booster. I don't recall ever looking for that switch, so don't know if adjustable like in my old Chryslers (slotted hole on clamp). It isn't adjustable in my minivans.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans