Are you sure that actuator ( its actually called a vacuum element) is leaking or not holding the vacuum. If so, its usually due to a leaking or broken rubber diaphragm in that element. You might not want to change the whole actuator. You can get the rubber diaphragm from ************************ or from
http://www.autoluxmbz.com/
Call them up they will tell you if they have one for you. They are about 5 bucks. Some people have tried patching up the diaphragm hole with a rubber patch and a rubber glue.( just like fixing a flat on side wall of a tire). But I would rather replace the whole rubber piece.
Also You want to see the ACC control manula to see how this element works. I mean when it opens or closes or if it paritially opens under certain conditions. Make sure that you do have a leak before going thru the whole trouble. To check fot the leak apply vacuum to that element and see if it holds the vacuum if not its leaking.
Good luck