I've posted two threads with a similar problem. One on the diesel discussion and one on here.
My 77 300d is also losing its power assist. It has vacuum and the boost seems to be consuming vacuum as the brake pedal is pushed since the vacuum guage drops 3 or 4 inches as the pedal is pushed, but there is little to no power assist. In other words, everything appears to be working, but it isn't.
I'd like to know exactly what the problem is.
It doesn't seem to me that the mechanic has given a clear diagnosis of the cause of the failure described in this thread. So far I don't see a clear reason to think the problem was the hose. What was happening with the hose to cause the problem?

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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