It may be that you just need new actuators in your doors. In my 81 300SD the rear driver's side actuator got into a bind when a mechanic worked on the (forget what it is called) door return thing that connects to the B pillar. Seems like the actuator was pretty cheap - about $25 but it has been a couple of years. Very easy to replace. I went through all the motions with the Mityvac and ended up replacing my trunk-bound pump (because I took apart a perfectly good pump to see how it worked and could not get it to seal again) along with two door actuators. My actuators were the only problem all along. All locks work perfectly now.