If I may offer this,changing your brake fluid every 2 years or so will prevent this in the future. The fluid pulls in moisture into itself. This in turn causes rust to form in the calipers . American manufactures don't tell you to change the fluid but the German ones do! If your fluid is black in color ,it has become contaminated by small rubber particles from normal wear.These particles clump in the lower parts of your system and this then attracts a moisture build up in this location that leads to the rusting and caliper piston sticking problem.