You might be thinking of the W100 which has hydraulic power windows and seat adjusters

Vacuum from the engine goes as far as the ACC system under the dash (line c in the vacuum diagram). The FSM section on ACC has that vacuum diagram but things are well protected under the dash. The parts that wear are the diaphragms in the vacuum actuators (or pods). You can get replacement diaphragms for the pods. The recirculation and defroster pods have two diaphragms each since they can maintain 3 positions. Those don't take well to replacement diaphragms. If the car is a keeper, get new pods to replace leaky pods. IIRC you can get to all the pods in a 126 without removing the dash. The center vent pod arm is riveted and I never solved the puzzle of how to remove it so I replaced the diaphragm in place (man, that was years ago). Get a Mityvac and test each pod.

There is a separate electric pump beside the spare tire in the trunk for central locking and optional orthopedic seats. I've yet to experience a central locking pod failure in a 126. I've heard of pump and lock switch failures. If central locking works (4 doors, trunk and fuel filler flap), there's nothing more to do.

Some folks replace the vacuum tubing under the hood with rubber or silicone vacuum line. That eliminates a lot of the fittings which tend to wear out the quickest. The hard vacuum line gets brittle but usually holds up until you touch it. The big case in point is the weekly thread stating, "I changed the oil and now the engine won't turn off with the key." Vacuum line only available in bulk in translucent white. You really have to twist the arm of the parts counter person to get replacement line with factory colored stripes and you'll pay for it. There aren't so many lines that color identification is imperative. Auto parts stores probably sell the vacuum line since they're used in VWs. You can use auto parts store fittings and bends as well but it might be hard to find the very tight bends. Get creative. Don't forget to keep the filters, check valves and dampers as they came from Sindelfingen.

Sixto
87 300D