I suspect that as a grey market vehicle, there has been an evaporative emissions system added.
I'm not exactly sure about an 85 model but many euro models worked like the diesel and vented to the atmosphere. The evap system of the time included a charcoal cannister which recieved vent gases which overcome a roll over low pressure valve (designed to keep fuel in the tank if the car was upside down). The cannister was open to the air at the bottom but the charcoal absorbed the fuel vapors to a point.
During driving a vacuum from the motor was pulled over the charcoal which pulled the fuel vapors into the engine to be burned. Cars of that era had a vacuum operated valve that opened when venturi vac appeared. Unless the cannister becomes sealed there would be no way for intake manifold vacuum to wind up on the tank side unless in was hooked up wrong.
Bosch Master, ASE Master, L1
33 years MB technician