trying to locate door lock main vacuum line . drivers side firewall area in engine compartment theres 2 check valves. and found another check valve under the dash drivers side tied along dash bracket where would you recommend to connect the mighty vac is there a diagram for 76 280c ??

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In my '81 300SD, there is a box of lines under the carpet in front of the front passenger seat. All the vac lines meet here and go to all the doors.

Not sure about yours but you might check there.