On my wife´s bike I have disassembled the front brake caliper, and I found that the brake piston has kind of melted together with the pad. I think that it´s some kind of piston-pad, or wear layer (I don´t know an English word for this) that´s damaged, but I can´t find this on my parts diagram. Does anyone know about this? Is it part of the brake piston when you buy it, or can it be bought as a spare part?