Well have you tried using a Use a 6 point socket? or the best way to remove it is to take your needle nose plier and open it up.Depending how wide
your pliers open up, you might not need to open it up all the way. Then just stick the needle nose pliers into the rear piston where the two grooves are, and push down while turning with as much force as you can push. My pistons seemed very tough to turn at first, but once you get going, it gets smoother as you begin to turn. The best way to do this, is to get the tips of the needle nose pliers to the far edge of the circumference of the piston as possible without going out too far (otherwise you'll slip out), therefore you get the most torque out of it. And essentially, the tips of the pliers fit into the two grooves and to turn the pliers clockwise. This is what I did when I have the same problem with my
Honda brake caliper.--And One thing,Please!!, don't ever buy the brake piston CUBE tool thingy, it'll be no different than flushing money down the toilet, not to mention the aggravation! Hope it works...Good luck...