Strange as this might sound, I have talked to a few guys who have noticed that the weight of the pedal itself, makes the brakes drag.
Is the return spring firm enough to pull the brake all the way back?
To test, devise a way ( bungee cord, person or support of some sort ) to pull the pedal all the way until it can not move any more forward and see if the rear wheels free up.
Hey, it is just a suggestion.
