I am speaking of my 1979 300SD. If I lock the car, within 10 minutes they are inoperable, as in the vacuum has leaked out. If I leave it unlocked and come back 3 hours later, I can lock the car with they key and have it work perfectly. But, if I go to lock it again, it will lock but not as fast, and if I keep unlocking and locking, it gets slower till no more central locks. It takes about 4 or 5 cycles till they don't work upon returning to the car. Coiuld that help narrow down my seach for a vacuum leak? Everything else works. Shuts off, climate control vents work.
Thanks
David

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2000 Honda Accord V6 137k miles

1972 300SEL 4.5 98k miles

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It seems to me that when you repeatedly cycle the locks, you are just using up the vacuum in the storage container so that symptom seems irrelevant to the problem.
So the problem seems to be a leak when the locks are activated. This could be in any of the doors, or the trunk lock or the fuel door lock. Is there any indication that one or the other of these operates slower? I know on my TD, one door was slow to lock until it finally failed altogether with a big leak. I'm guessing it isn't too difficult to plug the lines going to the trunk and fuel door. Do that and test the system. If it still has a problem, it is narrowed down to one of the doors.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

You have a leaking lock diaphram. Could be any of them, alas.

You need a MitiVac to find the leak -- all the diaphrams (there are four in each door on the W116) must hold vac indefinitely. If you pump one down and it slowly loses vac, that's the one that's bad.

The plastic lines run from the driver's door (yellow is the supply, yellow with green stripe is unlock, I think, with red strip is lock) to the other doors. Probably a rubber T in the lines right above the kick panel on both sides, plus a set at the center door post on the passenger side. There is one for the gas filler and trunk lock, too under the trim panel in the trunk. The filler door has only one diaphram, I believe, it's spring loaded to the unlock posistion.

You may also have a crack in the plastic line, most likely at the driver's door. If this is the case, you will only have a leak if you test the lines at the switch on the driver's door, if you open the lines and test anywhere else, they won't leak. Broken line can be fixed by replacing it or by using some rubber vac line to splice the broken part.

Diaphrams and line (opaque white only) are available from
Performance Analysis
1346 Oak Ridge Turnpike
PMB 258
Oak Ridge, TN 37830

tel. 865-482-9175.

Diaphrams were $5 each, plastic line $6 a meter when I bought some last year.

No affiliation except as a customer.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!