Have you thought of these???
http://homepage.internet.lu/customau..._v8.htm#BRAKES
Because, as you have found out, the rear calipers are crap.
see my thread:
http://www.classic-audi.co.uk/forum/...?t=7590&page=2
I got two 100s in a field in Wiltshire I am up there tomorow, both are non quattro though, 1990 models..... I am getting the wheels off one, so can have a look.