kz650 rear caliper

I have a 1979 kz650 C with the rear disc brakes....

I recently took a trip and noticed that my rear brake was getting a little tight everytime I would use it.... got to my destination and parked it overnight.

24 hours later as I was leaving tried to use the rear brake and nothing there.... I would press down the lever and it would not return back to its original position and barely felt any braking...
Checked everything out and when I finally got back home I took the master cylinder and caliper off the rear.. here is what i have found:
There was plenty of brake fluid in the resevoir and it was nice and clean...however the fluid towards the caliper seems as though rust is apparent. Everything else seems intact but...

I cannot figure out how to dismantle the caliper and the master cylinder... the manual doesnt help much either. I have them seperated from the bike and all but would like to rebuild them and check everything out.

I am thinking that it has to be a stuck piston?? maybe??
Also where can i get a replacement caliper/master cylinder for a reasonable price???

Thanks! All help will be appreciated!

You may want to check where the brake pedal pivots in the frame. Corrosion can get in there and cause the pedal to stick.