The central locking repairs are completed. Followed your suggestions and located a number of worn, brittle and cracked connectors. All have been replaced, as well as the bad vacuum actuators.
Final results: All doors lock, as well as the truck and even the fuel door. After eight hours at work today the system retained enough vacuum to cycle the locks 6 complete cycles.
Apparently the fuel door had been disconnected because the catch tab was out of alingment and the locking pin would not engage. All that was required there was loosen the two phillips head screws and adjust the tab outward.
I even figured out why the driver's door key would not lock/unlock. For unkown reasons someone tightened the adjustment screw, forcing the lock mechanism out to it's extreme. Backing out the adjustment screw, and matching the adjustment to equal the passenger side cured that problem. I now have a Benz that can be fully secured.
73 280 SEL