Features of the 2013 4-piston XO.
A-Bleed port.O-rings in this will fail in a quick manner causing this to leak.
B-Fluid opening.This gets clogged with shit such as dirt,pieces of the brake hose,and o-ring bits causing the pistons to cease movement.
C-Polished piston slot.Probably sized wrong,so fluid will get through and work it's way onto the pads.
D-Piston O-ring.Will get eaten by the DOT 5.1 and leak.Ending the brake's life.Usually happens within 6 months,or right when you open the box.