I just bought a 79 300 TD . Stiff brake pedal . I searched for info also . I read one post that talked about checking the o-ring . I loosened the master cylinder and heard a vacuum seal being broken . Hmmmm , still vacuum in the booster . That confused me more .Tightened it back up and the brakes worked well for a short while . Then ... I read the brake booster section in the Haynes manual . They say that the vacuum is built up on both sides of the diaphram and it is broken on one side when the pedal is pressed . To test it they say to press the pedal down a few times while the engine is not running to break the vacuum then, while your foot is pressing down on the pedal start the engine and if the booster is working the pedal will be drawn down a short distance to the floor . I just tried this on mine and the pedal was drawn down .I ordered a new booster earlier today .That will be my $245.00 test .Anybody else have anything interesting to shed some light on this? I have feeling it is the booster . Spike.
P.S. Since this is vacuum related I must mention my door locks work but not after sitting for a while . I doubt this is related to my brake problem but being new to these cars and this type of vacuum set up it wouldn't surprise me .