I had a rear sticking disc that plagued me for a long time, suggestions offered and tried were. Mostly recommended in this order...
0) Check your master cylinder, gunk from old brake fluid can clog up a very small return hole in the m/c causing fluid pressure to build up in the caliper.
1) You may need a new caliper seal and rebuild kit. a bad or kinked seal can result in a stuck piston.
2) Brake line, could have collapsed inside and thus choked the return path of the fluid.
3) After checking all of the above, I finally realized that the aftermarket master cylinder rebuild kit was not the right one for my bike (even though it was advertised as so) and hence had to shell out $40 to a dealer for two pieces of rubber no larger than a penny and a spring
... yes very high tech stuff to blow away your money on.
In case you decide to replace any part, do a complete teardown and rebuild. Instead of shelling out to a dealer for a new m/c rebuild kit, i recommend purchasing a universal front master cylinder assembly for about $50, of course dont jump into that yet, teardowon your system in the above order and see if anything is amiss.
Be sure to use the proper tools, if those snap-rings/clips get bent, you will have a tough time putting everything back together.