I bet your calipers are sticking partially, possibly causing them to drag a tad. Two toy trucks that I do maintainace on have had this problem (mine and my neighbors). They are 4 piston set-ups and one or both of the inside pistons has frozen up on both trucks. The brakes still seem to work ok, but the outside pistons are doing most the work and loading the rotor unevenly. Reman calipers are only like $40, I'd check this out before I put yet another set of new rotors on.