Here is a good document on Vacuum door locks.

Be aware that you have a check valve under the dash "if" someone has attempted to pull the yellow lines further out from the firewall they will disconnect it.

Also my euro had a cracked in the main supply line to the master switch. My 78 had a rubber coupler in the door jam to the master switch...on the 78 when the door was open it held vacuum...closed it would not...the line was coming undone from the coupler.

https://www.ifixit.com/Guide/Mercedes+W123+Vacuum+Lock+System+Diagnosis+Technique/20201

__________________