If your central vacuum locking doesn't work on your older Mercedes, These are the first things I would check.

I'm also making vacuum diagrams, and plotting to touch every single component in the vacuum system, which spans from central locking, hvac, transmission control, emissions, brakes, and even turning the engine off!

This diagram drawing is going to take some time, but it'll be well worth it.

Key points in video:
0:00 Intro, Vacuum diagram info
1:25 Getting access to fuel door actuator
1:52 Tall man crams into trunk
2:14 What we will be testing
3:52 Vacuum Reservoir leak test
5:05 Fuel door Unlock test
6:05 Fuel door Lock test
6:29 Removing actuator to repair
6:55 Explanation of failure and plan to repair it
8:40 Repair attempt #1
9:50 Repair attempt #2
10:55 Success!
11:53 Vacuum diagrams are in the making.

Here is a link to the troubleshooting diagrams and everything you'll need to diagnose and repair the problems seen in this video:
➡https://trythistv.com/w123vacuum/
➡Mityvac kit I use: https://amzn.to/3cm7k4p
➡Just the mityvac without the case and extra stuff: https://amzn.to/3v3Deei
➡More affordable option: https://amzn.to/3t3QlKL
➡Vacuum line: https://amzn.to/3cfK0oF
➡RC car shock absorber boots: https://amzn.to/3vbWt5C (46mm are the ones you want)

(Links used may be affiliate links, which could earn me a small commission if you choose to purchase via one of those links)