Maybe the threaded fitting is coming loose. Give the it a twist to tighten.

With the engine running, fit a vacuum gauge to the small nipple on the pump then to the nipple on the plastic check valve further along the brake booster line, then shut off the engine to try to isolate the leak. The stop level in my 87 300D stays down for days so I know the brake booster line holds vacuum for a long time after engine shutoff.

Enough older style pumps have become junk that someone should have a check valve to spare. Let me know how it turns out. Someone I know might still have the scrapped pump from the 93 SD.

Sixto
87 300D