Recently the passenger side door lock on my 124 stopped locking automatically. It would unlock fine with the others, but not lock. I noticed it moved a bit downward but wouldnt quite lock. Today I removed the door panel and checked the vacuum line, it looked fine. Then I attached a length of line to the actuator and tried to work it with applied vacuum and pressure, it was very sluggish and hard to operate, but it did hold vacuum. Then I noticed if I manually worked the lock stem several times, it would work automatically on it's own. So I cleaned and lubed the linkages and worked them back and forth a couple dozen times (with the vacuum line off). After reassembly the passenger door is now the first one to lock...every time.
It looks like this fixed it. Now I need to do the same with the others, and lube the window guides at the same time.
So if you're having trouble with the door locks this is a quick and easy thing to do first, before getting into testing the pump and lines etc.

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Brian
87 300Dturbo 180K #14 head still running R-12 SOLD 12/2017
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10 Sprinter 3500 chassis with a Class A Winnebago on it. 56K