I think you have the acc servo.
Has about 12 vacuum lines going into a block on the top of the unit?
If this is the case that whole block is released via one phillips screw that is accessed between the lines in the center of the rear of the vacuum block.
I am unsure about your reference to a bellows....
If it is the servo then make certain there are no cracks in the black housing or apparent leaking that may have taken place in the unit. This is the numberr one cause of failure is the plastic mid section cracks and leaks.
There are a number of tests that you can perform on your system prior to just replacing it and I think you might want to try those first...
There is also a doc circulating around that covers many of the simple tests.
Use the search function at the bottom fo the forum to search for "acc servo"
If you do not have the acc servo but instead have the monovalve then I have been typing nonsense...
Just kidding. If you haven't tried the search feature it will give youa wealth of info...
Good luck on the fix.