The bolts that hold the rear knuckles in place almost seem to be welded in place, and can be a pain to loosen. (And of course you have to get these off so you can have your rotors turned). If you do not have an impact wrench, then to make this job easier, do this. You will need a 6 sided 15MM open end wrench. and a sledge hammer. Put the wrench on the bolts and tap it with the sledge hammer. This will release the bolts. A regular hammer just doesn't seem to have enough mass to do this.
I had the same problem with that square tool used to turn screw the pistons back in, (just didn't quite fit), but rather than file the tool down, I used a drill to remove a little metal on the pistons themselves.
While your doing this, you might as well flush the lines with new brake fluid if you haven't done it in a while.