The W116 runs its central locking, trunk/gas-cap release, cruise control, and climate flaps off engine vacuum, and leaks are a common ownership headache one failure often signals wider vacuum trouble. A known diagnostic: with A/C on, flooring the throttle should not cut airflow at the dash vents; if it does, there is a vacuum leak.
Vacuum-operated central locking and accessory system
Mercedes-Benz S-Class (W116) · mechanical