brake hoses often deteriorate from the inside. often creating a one way valve inside. and they can often cause the pedal to be very hard to press.
if you don't know how old the hoses are, change them. the booster may be holding vacuum, but have a ruptured disc inside... I'm just tossing out possible issues you might have.
have you tested each and every piston on your calipers to be certain they are not siezed? debris can also cause hard pedal, as can a bad MC, although the more common MC failure is no pedal... or at least a falling pedal. get a pressure bleeder, or make one, then flush each and every caliper, and see if anything changes.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!