The new systems don't have a back-up pump the just use a pump that works both directions.
Your system uses vacuum to lock and to unlock. The new systems have only one line and use vacuum one way and pressure the other way.
You could replace all your vacuum elements a couple times for what the pumps cost alone. Fix your system and I would recommend replacing all the door actuators unless you know there history. The labor to find a small leak will cost as much as the labor to replace all three door elements. If you only replace a single leaker the system works with higher vacuum and the rest are subject to this and fail sooner.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician