I just bought a bike and the guy said the back brakes just locked up on him one day. So I took apart the brake system and cleaned it a little then bleed the brakes. Took it out for a drive and they were working fine for a while till they locked up on me to the point where my back tire dragged when I tried to push it. I put it in a truck and took it back to my place. By the time I got home they had loosened up enough where I could push the bike easily. I took off the caliper to try and see what was going on. I was able to pry the caliper pistons back in most of the way. I then pressed the brake a few times and noticed that after the pistons compress they dont retract. So I see while driving after pressing down on the brakes a few times why they lock up. Any help on why the caliper pistons wont retract after they compress?