I thought there was a sealing gasket that went between the booster and the firewall. Seem to remember that on an 80 model 300TD. That gasket may be your leak?
Please let us know what you find as the leak.

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Turbo300Mercede
87 300D W124
83 240D W123
80 300 TD Wagon W123, 4 Speed from 79 240D, SLS Rear Suspension