This is rather an old thread I've dug out, but maybe someone else has some experience since. My Multi has ATE calipers, which I think are universal. Behind the sliding pins are two plastic caps that cover 7mm Allen head bolts. I assume these have to be undone to split the fixed from the moving part of the caliper, but I couldn't move one of these bolts. Am I doing it wrong or do I just need to get a bit medieval on them?
The Girling calipers on my Alfa work in the same way, but have 13mm conventional bolts and you have to hold the pins with a 15mm spanner, which sounds exactly as the procedure described by Hellcat above.
Not sure why I mentioned 13 and 15mm bolts... That's the Tipo - think all bravo/a/marea/multipla had the slidey 7mm allen bolt thingies. Silly Hellcat.
How are you trying to undo them? With a 7mm allen key, or a allen socket? My experience has shown you need an allen socket to put enough force on it. They're a standard clockwise thread.