Have a 96’ disco that has issues with brakes. Brake pedal seemed to show signs of a bad master cylinder. Fade when applied. Bled the brakes.. no change..changed the MC.. Firmer pedal but still gradually fades almost to the floor before stop.The rear rotors look rusty and looks like there is very little contact upon braking. I thought it might be the rear calipers but I thought at least one would be grabbing more than the other...not the case.

 

Do the pistons of the rear calipers move or are they frozen?