This is interesting. I made many many calls a couple of years ago trying to find someone who would rebuild the brake booster on my car. All the answers were the same:
1) It is not rebuildable
2) It was not designed to come apart
3) Thus, because of 1 and 2, parts are not made for rebuilding.
Consequently, I'd be interested to hear of other experiences, or you might want to ask your shop how they did it, or where they got the parts to do a rebuild.
Michael Salemi
1969 280SL
Signal Red w/Black Leather
Restored