rear brake pads not in good contact with rotor

[mattn]

Checked my wife's car and saw the rear pads are not making good contact with the rotors. Very little of the rotor is shiny while the majority is "rusty". I am planning at this point to replace all pads and rotors but wanted to ask about the potential need to rebuild the rear calipers. It seems they are not moving even under pressure...

I have rebuilt calipers on my 1972 911 but I wanted to run this by mini experts. Have never worked on a Mini. so far.