Hi. I've built many bikes over the years (I'm 58) and have had this problem many times.
I've also had issues after fitting braided steel lines, and think the problem is very simple! When you drain the entire brake system, tiny air bubbles stay in the lines, and just will not come out. As you have experienced, no matter what you try, the brake remains spongy.
What I've always done is to bleed the brake as best you can, shaking and knocking the new lines will help remove air...but you just cannot expel it completely!
The very simple solution is to pump the lever a few times, then bind the lever so that the brake is locked 'on'. Leave it overnight, this will force the now compressed air bubbles to let go and rise to the top of the system...sounds a bit odd, but it works.
I don't claim to have any real reasons why, but this works every time, so who cares!