You say the other three door locks don't work. Does the fuel tank cover lock and the trunk lock work or not. If not, I would strongly suggest that you check out the distribution switch which is behind the driver door panel. It does not sound as if you have a slow leak somewhere else. It is the black horizontal tube near the lock with three tubes connected. Someone else mentioned slightl;y repositioning this to reduce hissing. Same advice goes to seeing if you can get it to operate satisfactorily. Last time I bought one of these new it was about $30, probably more now. The 123 service manual has a good section on the vacuum locking system. Haynes has little. I got a very good vacuum pump at Sears for $39. All I needed for this tracing. Alternatively, the MightyVac availavble at auto stores should do the trick. Very useful tools, either/both.

Whaile you are at all this, I strongly suggest putting two small air/vacuum filters into the system under the hood. these will fill with engine oil if you get a leak at the vacuum pump or the shutoff device, thus saving you Big Bucks replacing all the rubber and cleaning out the tubes. Not to mention the ignition switch! There were articles in Star several years back about these filters. I installed them accordingly, and within two weeks one filed indicating pump failure. Two weeks later the other filled, indicating shuoff device failure. I was very, very lucky to have caught the failures this way before the oil fouled everything.

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Douglas
1959 M-B 220S cabriolet