bought a used front 4 piston pm caliper and when I brake and release the pads are dragging. What's the problem and how can I fix? thanks

 

How bad? There's always a slight bit of drag until they 'break" in.

If not you may need to shim the caliper so it it more centered on the rotor.

 

it's so bad when I brake the pistons stay stuck like I'm still applying pressure.

 

Rebuild the caliper

 

Yup. Take that thing apart. Sometimes you can free up the pistons by working them a bit by hand.

 

damn I was afraid it was going to come to this. Oh well.

 

Is it lined up correctly? A slight offset could cause this too.

Take the pads out and remount the caliper and make sure your rotor is dead center. They are not all designed the same.

 

If its centered properly, the odds are good that its most likely the pistons sticking.
A used caliper will raise the odds of it having the pistons stick in the bores especially if it hasn't been properly rebuilt before its installed.

 

OP, in your second post you said that it's like you are still applying the brakes. This is a Master Cylinder problem. Even if the pistons in the Caliper are sticking, the worst problem you will have will be some excess drag and maybe a bit of noise after you release the brake as the Brake Pads are still dragging on the Rotor face. But it will NOT be like you are still squeezing the Brake Lever.

By any chance was the Master Cylinder or the Brake Lever worked on; either before you replaced the Caliper, or when the work on the Caliper was being done?

When the Brake Lever is released, a Brake Master Cylinder has a passage that opens the brake line to the Brake Fluid Reservoir. This passage is called the Compensator Port. The Compensator Port allows the Brake Fluid that was pumped into the Brake circuit when the brakes were applied, to be released. It also allows Brake Fluid pressure to return to zero in a Disk Brake System. When you squeeze the Brake Lever, the first few thousandths of an inch that the master Cylinder Piston moves, closes off the Compensator Port. As the Brake Lever is moved further the Brakes are applied because the Brake Fluid is now trapped between the Master Cylinder and the back sides of the Caliper Pistons. The Caliper Pistons then push the pads against the rotor. If the Master Cylinder is not fully releasing, and it doesn't have to be by much (we're talking thousandths of an inch here), once you apply the brakes you'll either be waiting a very long long time for the pressure to drop (if ever) or you'll have to open the bleeder screw or crack a line to release the pressure. If this is what is happening on your bike; once you apply the brakes . . . they will be APPLIED! You may be able to pump the Lever once or twice more and make the problem even worse, but the brakes probably will not release until you open the bleeder screw or crack a line as mentioned earlier. There should be just the slightest amount of movement in the Brake Lever before it starts pushing the Piston Cap and it in turn starts pushing the Master Cylinder Piston Assembly.

The three illustrations that I've attached show a Master Cylinder with the Brakes Released, Brakes Applied, and one with the problem you are having.

An easy way to see if the Compensator Port is being uncovered as it should be, is to: 1.) Remove the Master Cylinder Cover, 2.) Apply the Brakes, 3.) Release the Brake Lever, 4.) Observe the master Cylinder Reservoir. 5.) If the Compensating Port is being uncovered as it should be, you should see a small spurt of fluid returning to the Fluid Reservoir when you release the Brake Lever. If not . . . that's your problem!

 

when i got my pm 4 piston front caliper it came with some shims,if yours isnt centered correctly, this maybe a problem!
you could try draining brake fluid from master cylinder and bleed some new fluid in!