You may have a ruptured diaphram in the booster. Does it hold vacuum with the engine off? You should be able to apply the brakes with full boost a couple times before the vacuum all gone -- if you don't have at least one application's worth of vacuum in the booster after you shut the engine off, it is shot.
Poor braking could also be air somewhere else, too -- bleed the whole system to replace the fluid if you haven't done so.
If you have a high, hard pedal without vacuum, there probably isn't anything wrong with they hydraulics, unless you don't have the seal between the master cylinder and the brake booster.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!