Hello,

I imagine that one of you is reading the title and immediately thinking "obviously its a vacuum system issue, classic W123".
So a couple days ago I noticed that the doors were not locking on my 1977 240D.

I can put the key in the drivers side tumbler and feel no resistance as I turn the key CW and CCW, this does not actuate the door plunger at all.

On the other side (passenger), I put the key in the tumbler and feel it get "stuck" as I turn it CCW to lock it, still will not actuate the plunger, but I can see it twitching as I try to lock it from the passenger side.

When I am driving, say, on the highway, and the engine is running, I can push the plunger on the drivers side door down and it will lock all the car doors. As soon as I turn the car off, I can no longer manually actuate the doors.

I am pretty sure this is a vacuum leak, but I have not dove deep into it yet. Has anyone had experience with the symptoms I am experiencing who can point me at either a part or a section of vacuum tube to check?

ALSO: What is the best tool for diagnosing vacuum problems? Is it a simple hand pump I can get off amazon? Should I splurge for an electric vacuum pump? let me know.

thanks,
Jhu805
1977 240D 4-spd Burgundy Red