I'd been trying to figure out why the brake pedal effort on my 77 300d was so much higher than the pedal effort on my 85. Turns out it was worn front brake pads. Brake wear indicator had only flickered very recently but had noticed the high effort back in January when I drove the 77 after driving the 85 for a long time. Immediately noticed it took twice or three times the effort on the brake pedal. Initially thought it was bad booster or vacuum pump. Rebuilt the vacuum pump. No difference. The brake light wasn't coming on and I had about 20k on the front pads, so I didn't think it could be a pad problem. In addition, I still can't figure out why worn pads increases the pedal effort.
But they must. I replaced the pads this AM and pedal effort returned to the same level as the 85. Pads were worn down to the indicator drill hole with about 1/8" left on the thinner outside pad.

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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