To get these pistons in all the way you not only have to turn them in but also push at the same time. There is a special tool for rear disc brakes that is sold at most auto parts stores. You are on the right track by turning them in but if you are not pushing them in at the same time like you would do on the front ones they will not go all the way in and you could damage them inside . I am going to try and send the KIA service manual procedure with this post but because of copy rights sometimes they won`t copy, hopefully it comes through.
Best of luck,