I put end caps on the new cables I added, put the bike on a stand and checked the rear brake. It was binding at one point in the wheel's revolution, and after a bit of fumbling around, I realised that the wheel wasn't true.
A true wheel, when rotating on its axis, and looked at from behind, doesn't appear to wobble from side to side, or up and down. You true a wheel either with a special and rather expensive wheel truing apparatus, or you do it in the forks. The apparatus just shows you where the wheel isn't true; you get exactly the same effect if you rotate it in the forks.
This was the new wheel with the 11-28 gears, and I was surprised to see that some of the spokes were loose. I had assumed that it had been trued before they sent it, but there's no good reason for that assumption. Truing a wheel takes time and a bit of patience, and time's money. So I got out my spoke spanner, and pinged the spokes. You can tell when a spoke is slack, because the ping is either very low pitched, or if it's actually loose, it doesn't ping at all. So I tighened up the ones that were loose, then the ones that were slack, then I worked on truing the wheel.
The Holy Bible for bike maintenance is Sheldon Brown, and he's done a good article on wheel truing, so I won't explain the process; read Sheldon. To be more precise, Sheldon is like Moses, his writings are like the Bible.