I think that the brake is dragging a bit too much on the rotor, and I think it's because the piston isn't lubed enough. I recently went through a tire change, and when I had the brake off the rotor, accidentally squeezed the lever, and ended up wiping some of the grease off the side of the piston when i was trying to get the pads apart.
Anyhow, my shop manual says that I should use PBC (Poly Butyl Cuprysil) Grease. From reading around, it's hard to find, and expensive. I'm wondering if there's a replacement that could be used in this application instead without causing problems.