The brakes on my 77 300d require MUCH higher pedal effort than the brakes on my 85 TD. I have all vacuum components disconnected except the ignition switch and the brakes. The vacuum is at about 18" which I think is appropriate for my altitude (5000'). I can also pull a 20" vacuum on the brake booster and it holds it. Vacuum seems to respond approrpriately to brake use. Drops a few inches for each use, then rebounds. Rebuilt the vacuum pump a few months ago.
Why would the pedal effort be so high? Jacked up the car and both front and back brakes (on one side at least) seem to be performing correctly.
I'm pretty sure this big difference in pedal effort between the two cars is a pretty recent phenomena, although I only noticed it after driving the 77 after not driving it for a few months. (My wife usually drives it)

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13