Hard (power) brake pedal

The following assumes that your hydraulic system is fully functional. If your caliper pistons are jammed tight to the rotors or the rear drums are over-adjusted, or Combination valve is stuck, or the Flex-lines are not passing fluid, or something is not plumbed correctly or the master cylinder is defective; then you gotta fix that first.
Ok
with the hose verified to be good, not plugged, and properly plumbed from intake manifold to vacuum chamber;
engine off, evacuate the booster by pumping the pedal about 5 times. Now step on it moderately with something like 50 pounds of foot pressure, just guess. Then start her up and wait a couple of seconds; the pedal should fall about 1 to 1.5 inches, by itself. Maintain the pressure. If it doesn't drop, rev the engine up to something like 1200rpm, and wait a couple of seconds. If still no drop, relax and shut it off.
Now, the diagnoses is that either the pushrod is too short or the booster is shot.
Pop the hood, and at the booster,by hand, pry the checkvalve over to the side and listen for air rushing into the booster. You do have a checkvalve right?
If no air rushes in, then put the valve back, start the engine, and let it idle. Get a helper to press the brake pedal down just a little. Grab a long nosed plier and squeeze the hose tightly shut, wait a second or two,and observe the engine idle quality. If the engine smooths out with the hose pinched, you can bet the engine is sucking air through either the diaphragm and/or the control valve, in either case the unit is no good.Some can be rebuilt. If the control valve is broken, it's probably because the pushrod is too long and someone accidentally hammered on the pedal too hard. You will have to investigate this before you install a new booster.
But if air rushes in then both the diaphragm, and the control valve are likely Ok; leaving the pushrod adjustment.
For a hard pedal the pushrod is too short. Just unbolt the booster(see edit1 below) from the studs and slide it towards the radiator just far enough to peek in there behind it. There you will see the pushrod with it's acorn-nut adjuster. Your job will be to make it one turn longer, but you have to anchor the rod, IIRC there are flats on it for a small wrench. IIRC that threading is "normal" so CCW from the engine side should do it.
Now bolt it all back together, and retest.
If still no drop, make the pushrod another turn longer and retest.
Repeat until the nut falls off the end lol. If this happens to you; either the pushrod is wrong or the control valve is outta sight, or there is something wrong with the pedal system. I'll bet the booster is bad.
The reason you only go one turn atta time is because if the control valve is ok, and if the pushrod gets to be too long, and if you mash the pedal down too hard; then it is possible to break the control valve and instantly you have ruined the booster. Don't let this happen to you.
If/when you start to get boost,then you can fine tune the amount of boost playing with the pushrod length until you like the way it modulates.
But if the booster goes from no boost to wham, then the control valve is for sure defective, and by definition, you need a replacement booster.
Count the turns as you go, cuz if the booster gets replaced you will want to shorten the pushrod back to where you found it... at least.......... so you don't break the new one right away, on the first panic stop.
That's my best shot

Edit 1; unbolt the M/C from the booster; sorry.