FSM : on checking brake booster page says:
Overlap between push rod piston of main cylinder and push rod of brake booster:
0.2 to 1.2 mm.
this means push rod on brake pedal/booster moves the main cylinder piston by that much , at rest, ie before pushing the brake pedal, right?


Mine has unsatisfactory gap / slop, so the pedal requires travel before it begins braking.

Now it's drivable but I would NOT take it on a long trip or even beyond local in town driving

After:
new ATE master,
endless bench and caliper bleeding (ad nauseum)
all new hoses
rebuild of rear calipers

still the pedal is:
variably firm, still soft enough for concern
supports only one or two pedal pushing before loosing power braking

vacuum pump:
delivers steady 20 in. Hg at idle and at revving with gauge directly on its output
vacuum consumers side branches deliver only 1 to 2 in. Hg at idle and 7 at revving
the booster sees no more than 12 in. Hg at revving with gauge on the disconnected from booster check valve tube