I recently replaced the brake master on my w123 with a reman master and had same issue afterwards.

Turned out they hadn't supplied the o ring that goes between master and booster. I didn't notice because I'd given my old master (and o ring) as a core.

Try blowing smoke around where the master meets the booster with engine running and see if it sucks it in.

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1984 300TD - der Landstreicher