It might all be related with the brake booster being the culprit.

However to isolate the inability to shut off car, try turning off the car in either the door locked and the door unlocked position. Or just get under the hood and disconnect the yellow lines from the 4way and plug it with a golf t. If that works, you have a leak in the central locking system.

But again, your braking issue also sounds like a booster issue which is also related to the vacuum system.

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1985 300d Turbo Diesel