My 1975 [116 model] is having heart attacks with the pressurised central locking on all doors. It has a leak because it won,t hold the pressure in the lines at all,it is getting worse, now i am down to 2 doors operating only [both the front doors]
I have checked and replaced a couple of the rubber seals in the individual door units themselves but to no avail. with the engine running, there is still not enough pressure to completely lock or unlock the rear doors. I have replaced the 'one way' valves under the hood but that was a waste of time also.
Any ideas as to how to diagnose where the leak is without replacing all the door and trunk units? I,d hate to do this and then find my problem may be 1 of the lines themselves. I can install an electric system like all modern day cars, but my preference is to keep it all original
any help would be appreciated

mrkozzy,

If this were a 123 car I'd be ready for you. You should be able to find a vacuum line schematic. With the schematic in hand and a vacuum tester from the Auto Parts store, you should be able to trace your problem. If it's anything like a 123, start where the supply line goes through the firewall. If the whole door lock system won't hold a vacuum, go halfway, which is probably the line leading to the right doors and try there. If it holds a vacuum there, go back toward the source, if it doesn't go to the back door. Continue splitting the system you've tested in half until you've homed in on it.

Good luck,

------------------
Larry Bible
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles

It worked
 

Did it exactly the way you said Larry and eventually found the culprit to be the line to the locking petrol flap. Took a while to find, however your system was always going to work, as it was methodical.
Sorry it took all this time to post a reply.

Thanx once again

yikes! an update 7.5 years later;)

glad it worked for you mrkozzy