I failed to recount an important lesson I learned on my 77 300d. My brakes were very stiff. Vacuum booster checked out, so I rebuilt the pump. No difference. Turns out the little spring steel pieces that hold the front pads in position were stopping the calipers from squeezing the pads against the caliper, resulting in high brake pressure. Put in new front pads, brakes returned to normal.
If this is your problem, it doesn't account for the hissing sound you are hearing. What are your exact vacuum readings under various conditions?

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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