Glad you got the ABS light to go off. It never hurts to bleed the system, but do the entire system, starting with the front, then the rear. If you're really feeling the moment, do the clutch as well. Normal DOT 4 fluid is best.
The pulsating can be warped rotors, but also just pad build up. Pad material can build up on the rotors and it will give that same pulsing feeling. You can sand the rotor with some emory cloth or just some 200-220 grit sandpaper and work your way around both sides of the rotor. Ideally in an orbital motion. Not so hard that you are rounding off edges, just staying flat to the rotor and work your way around. This will remove any glazing and pad build up. Then go for a ride and see how the rear brake feels. If no change, probably warped. Some live with it, others replace the rotor. Brakes still work.
Warpage usually happens when the rotor gets very hot, then you stop and hold the brakes on for an extended period of time. Sometimes a long traffic light at the bottom of a long hill can be enough to cause warpage if you're not careful and had to use the brakes a lot on the way down due to traffic or other conditions. The pad cools that spot on the rotor faster than the rest of the rotor, which creates the warp.