The door locks on a '78 are operated from a swith on the driver's door -- the yellow plastic line goes to the vacuum swith there, and the lock/unlock lines (yellow with red stripe and yellow with green stripe) go from the driver's door to the vacuum elements in the doors, the trunk latch, and the gas filler door.

It sounds as if you have vacuum applied to the passenger door element all the time, or that one of the lines is plugged.

You will have to remove the door liner to get to the locking element -- pull the lines off and use a mitiVac hand pump to test. If it doesn't work, replace it.

You can also check the operation of the system with the mitivac by connecting it in the engine compartment and manually pumping down the door lock system.

My guess is that you have oil or dirt (or bits of a failed check valve) in the locking system -- if you have oil, the vacuum pump diaphram is bad and you need to replace it, and clean out all the vacuum lines.

The little blue thing is a check valve so that the system doesn't leak down when the engine is off -- you can replace it with any similar check valve if you don't want to spend the money on the MB part.

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!