The FSM for the W114/W115 models is located
here for your future reading pleasure to help you become better acquainted with your 1974 240D W115.
(Factory designation 115.117 - also the first six digits of your VIN).
Rebuilding the vacuum pump is in
FSM section 42-620...in case the diaphragm is torn in your pump.
MB has used vacuum controls in a lot of different systems, ignition switch/engine shut off, brake booster, central locking, HVAC, cruise control, headrest positioning, orthopedic seats, headlamp positioning, transmission shifting.
Fortunately, not all these apply in your case.
The door lock vacuum diagram without AC is located in the
FSM section 80-900
The door lock vacuum diagram with AC is located in the
FSM section 80-901
These diagrams are not showing the entire vacuum system...but it's a start.
Hopefully other can chime in with additional help.