I posted this issue on the diesel forum but I thought I'd try here too. I have a 77 300d and 85 300td. It's taking 2 or 3 times the pedal effort to stop the 77 compared to the 85. This situation seems to have slowly, over time, gotten to this point. My wife usually drives the 77 and I drove it the other day and realized there was a problem. I'd almost say it seems as though the 77 is not getting any boost at all, or very little.
The vacuum system seems to be ok. Holding about 18" (we're a Mile High here). Vacuum booster seems to consume vacuum appropriately, using 3 or 4 inches for each pump. System vacuum recovers from brake usage in a few seconds with engine at idle.
Can brake boosters fail without leaking vacuum? Anyone have any idea what might be happening?

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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

With the engine off pump the brake pedal 15/20 times,then while keeping your foot on the pedal start the engine. You should feel the pedal go down under your foot as boost is applied.
If the pedal goes down check the pads .
Mike