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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?
 

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Check the master cylinder reservoir; there is a hole in the bottom, and maybe two. If only one, it does two things - refills the cylinder as needed, and releases pressure from the system when the pedal is released. If there are two holes, one is very tiny, and is the one that releases the pressure. If that pressure won't release, each time you use the brake there will be more pressure remaining on the caliper pistons. Make sure those holes are clear, and see if the problem doesn't go away.
 

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Any help on why the caliper pistons wont retract after they compress?
Very likely that it is dirt/corrosion keeping the pistons from retracting. The pads retract a few hundredths or even thousandths of an inch from the rotor, pulled back both by the stiction of the O-ring seals on the pistons and by some slight fluid pull back of the master cylinder. When the pistons get coated with some dirt or corrosion they can't pull back into the bores, the fits are very tight. This can happen on off roaders from riding through deep water and mud, but on road bikes it's usually caused by pressure washing. Even a garden hose focused on the brake caliper with any forceful spray can push water under the rubber piston covers and force dirt or cause corrosion in and around the caliper pistons.

You'll need to pull the caliper, pop the pistons, clean them, and I'd install new seals since it's apart. Then don't blow water in around them with any force.
 
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