Just the caliper, and for some models, a longer brake line. The logic of the master cylinder is that it (may) give improved rear brake performance. Since there are many different diameter master cylinders out there, I tried to find out 1) what the OE MC diameter is, and 2) what the diameter of the matched Ducati/Brembo MCs were. I came up empty. If anybody knows, or finds out, post up here &/or send me a PM.