Does it build up pressure if you pump the pedal? I am assuming this is the '85 300TD.
Do you see brake fluid on the inside of the front or rear wheels/tires or hoses? Is there signs of a Brake fluid leak around the Master Cylinder, Brake Booster or lines attached?

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Collector of Fine Hose Clamps
'77 240D 'AVA' is in heaven now
'80 240D Kanarienvogel
'82 300TD 343k was my daily driver 'ADOLPH' In Surgery for a severe Deer Bite to the Nose
'88 560SL 102k 'White One'