Once every month or so I will go to lock the doors in my SD and they will not all lock. I can solve the problem by disconnecting the battery for a second to reset the vac pump, but I would like to know why it is happening. I do not have any leaks in my door lock system and the locks function normally and quickly when they do work. Any idea what could cause this?

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine