Cruise Control - 560SEL

Hello Again; I have a 1988 560 SEL. The cruise control is not working properly or sometimes at all. I talked to a mechanic re this and he said something like a stuck valve in some vacuum can??? associated with cruise control. He also suggested that it may be the printed circuit board that controls the cruise control. Any idea where these items are located on this car. I don't have any manuals of any type yet.

Thanks, Timberock