Yeah, I've had the same problem with my front hope, it is actually caused by a sticky piston even if it doesn;t really feel like it. What I did was take the wheel out, squeeze the lever so that the sides of the pistons are showing, take the brake pads out, squirt the outer part pf the cylindrical pistons on either side with some WD40, use a screw driver to pry them open again and repeat a few times. That worked for me both times just be careful to wipe your caliper after as for some reason no matter how careful you are, oil seems to be desperate to infect your pads and cripple their performance .