Lots of good advice above - especially with regard to vacuum checks with hand pump - (if you don't already have one, can get a cheapie at most auto stores; consider getting one with the reservoir cup which enables you to do one-man brake bleeds, as well)
NOTE: "pulling" hard plastic lines out of connectors/hoses is usually counterproductive and may result in breaks/tears - the rubber tends to shrink/tighten when pulled upon - rather, hold the hard line and use a flat-blade screwdriver or similar tool to PUSH the rubber hose end off the end of the hard line...
Easiest initial access indeed at manifold "Gumbi's" - red to front reservoir, black to cruise control, yellow/grey to CLS front door switch/rear reservoir, blue to seat locks (coupes), green to climate control...
next at trunk unlock panel - yellow/red to "lock" trunk and T'd at gas cap lid;
yellow/green to unlock trunk (gas cap springloaded to unlock if "lock" released)
The connector rubber hoses and hard plastic lines are incredibly robust and long-lived - [EDIT] [the gas cap actuator is fragile and small and a suspect leak culprit]. If it is, it's a little awkward to reach - in my 280CE/E's, the mounting screws are, however, in a keyhole slot - just loosen and lift out (no need to unscrew them out).
You can see if there is any difference in leakdown while unlocked, as well as while locked.
If losing whilst locked, but not unlocked - I would check gas cap's actuator first. Both the gas cap's and trunk's are easy to access, and not nearly as robust as the door locks' bellows. [EDIT] [One] can also get at the lock and unlock hoses alongside rocker-side floor pan edges, as well as under the rear seat. [EDIT] [the hard lines are continuous from the firewall, but you can cut in and test, then splice, with hose.]
Pull up typical system in:
http://skinnerbox.steaky.org/Service/W123/Main.html
under M110, any similar model, such as 280E -
Chassis/Body Manual
Central Locking System (section procedures 80)
80-260 (troubleshooting/checkout)
80-900,901,902 (system diagrams)
{EDIT}
Litton, glad you've been able to isolate to doors; if you're limited in resources or stubborn, you can CAREFULLY dismantle the membranes or either side out of junkers and repair/replace a single torn ones' elements, but you still have an old membrane, and you probably love opening up doors as much as do I...also, if you find good used dual elements with different pull rods, you can judiciously PB-Blaster the rod end in the potmetal "bridge" and pop the rod out of the notch with channelocks. If replacing whole used elements which used different mounting holes, or new, recommend pre-threading the sheetmetal screws while out on the bench.
Also check replacement elements - I gather there are at least two versions, which may or may not be interchangeable.
Stan.