Looks like that's the issue. The bolts are 12mm, not 13mm or 1/2 inch. Now that someone has pointed that out, I remember that from when I pulled the calipers off my P38 to do the ball joints. Try to use a 12mm 6 point on them. At this point you may need to hammer the socket on so it might be worth picking up a 1/2 inch drive impact socket and breaker bar. Beat the socket on the bolt and get after it.