since I got a few PMs about the bigger boosters, here is most of my knowledge.
this is only info copied from german forums. no guarantees. not tested here yet.
part numbers:
ATE 03.7760-0902.4 Bosch 0986 485 113

picture tutorials
http://www.t3-infos.de/blog/?tag=bremskraftverstarker

http://www2.vwbus-online.org/forum/forum_entry.php?id=656010

if anyone completes this before I do, please let me know.
FLorian