For me this usually mean the piston is corroded or filthy and it needs to be cleaned.
If you are capable of bleeding the brakes, then you should take the caliper off, pull the pistons out and clean everything up, then reassemble and flush/bleed the fluid. Check the pistons and pots for rust or corrosion and clean that up too while you have it apart.
Also make sure you have the pad spring installed and that the brake pad pins are clean and the pads are able to slide on them.
Basically just take the caliper off and clean everything up... It may sound like alot but it really isn't bad. It can be annoying and difficult to get the pistons out, but compressed air can help there if you have it. If you can't get it, you can hold one piston in witha small c-lamp or something similar adn use the fluid to push it out, clean it up, re-install, then clamp that one in and force the other one out and repeat.
2002 DL1000: 75k miles. Bought at 50k miles.
1992 XJ600: Red, Very custom.
2002 Vespa ET4 150: The wife's ride.
Last edited by Nelsonmd; 06-21-2016 at 10:53 AM.