cyberprais

I am a new owner to an 86 300sdl. It is a great running car, and almost everything works on the car but the cruise control . The cruise control worked one time a few weeks ago when it worked only in acceleration mode. The speed control would not stabilze with the steering coluum lever. The car continued to increase speed until the brake pedal was depressed. Since then all attempts to activate the control have failed. any ideas of where to begin? The vacuum pump generates about 20 units of vacuum.