One of my 79 300CD's has not been touched in four years, or more. The car just unlocked, and re-locked with no problem, when I sent my father out to the garage to test it for kicks. Why? Does my car have a different locking mechanism? Perhaps, not vac system, but mechanical intead. Is it possible, it's still maintaining vac. pressure after all these years? I know it's got the tumbler type locks on it, by the inner door handles, and not the pop-up type locks on it. Is there something different about my car?