There is not supposed to be ridges in the rotors.
May I suggest you disassemble the rear brakes and see if the pistons can be moved.
Typically, there is around .002 to .004 inches of space between the pad and the surface as long as the square cut seal is working properly. When rust/corrosion prevents the piston to return, the pad rides on the surface until it wears down.
Just a suggestion.
