I've had several of the wall buttons break, the screw pull out, but never a string break. And like someone else said they are a pain.
If I remember it correctly, I pushed the shade to the top, removed the other string from the other wall button, and pulled the plugs out of the ends of the pull bar. Pulled the loose string out the other side and tied an "extension" string onto it.
Ran a piece of wire through, tied the string onto the wire and pulled it back through.
I didn't tie the string back to the buttons, I put a loop on the end of the strings and slipped them around the whole button.
They are pretty tight but it wasn't too hard.
