Bleed the rear brakes. I bet some air is in the system. Don't know why air seems to get in these sealed units but they do. My Rocket had the same problem. After verifying I didn't have a caliper binding up, I bled the rear brake and sure enough, some air came out. No visible leaks either. All ok now.
PS, You should flush both sets of brakes every couple of years with fresh DOT 4 brake fluid. You will notice a difference in lever feel/feedback.
