1991 Mercedes Benz 420sel Brakes

How difficult is it to replace L & R rear calipers, pads and sensors on this car? I have done this on an early model Volvo but on this car I do not know. Right now after installing a new master cylinder, the rear
caliper pistons that hit the disk now do not withdraw completely and consequently it feels llike a slight drag while the car is moving. The sensors apparently are constantly in contact because they are melting. I have tried lubricating the pistons that fail to withdraw once your foot is off the brake but they fail to do so. Any suggestions as to how I can correct this problem? I guess the new master cylinder is exerting too much pressure on the old calipers. Can removing some of the brake fluid help the situation?