Ok, so i changed my front brake pads because they were making noise.. they ended up only being 1/2 worn. Had them off and had already bought new ones so i replaced them anyway.. have been working fine for 2 weeks now.. and today it starts making a crazy loud screeching/rumbling when stopping.. so i take it all back apart and see the the metal ring around the face of the caliper piston has a piece of metal that has sheered of, and is sticking out between the piston and the pad.. so i gently remove the piece and lightly sand.. put it all back together and is working fine... but what the hell coulve caused that.??? and now i feel i should replace the calipers. Or at least the piston if i can get those alone.. .. any thoughts?? Why is there even a metal ring around the face of the piston??