I stripped off all the interior panels and trim from the car's interior so I could get at the Vacuum Lines. Need less to say my carpool wsnt thrilled about this in 10 degree weather.
The System works fine up to the Front Driver side door.
I isolated the driver side off the car by plugging the lines running over to the passenger side at the Tees near the front seat.
When I lifted the Driver side front door lock up and down I could see a faint response in the driver side backdoor. The rear driver side lock locks correctly in response to my locking the front door driver lock but does not unlock.
There is a lot of binding in the driver side rear door locking mechanism and it takes some force to raise that rear driver side lock.
I am suspecting the large vacuum element that runs the rear door lock. Is it worn out and does this happen in older 240D's.?
A lot of fellow owners tell me that their vacuum locks no longer work

If your 1983 vacuum system is alike mine from 1979 I might be able to help:

A possible source of failure could be the vacuum-/keyswitch in your driverīs door. It should be removed and checked with a Mityvac vacuumpump.

If you suspect a vacuum module in your rear driverīs side door it should be removed and the rubber diaphragm should be re-glued at where the ring sits. With the help of the vacuumpump you can put it under water like you mend a flat bike tire and look for bubbles.

You wrote your system works fine up to the front door. Just make sure that the vacuum valves under the hood are intact. Again check with the vaccum pump.

Regards,

Jorg
1979 MB 450 SE
1979 MB 450 SEL 6.9
1991 MB 300 SL