Help! What has happened?

[MachAzure]

I was changing the right rear brake pads and was going to turn the piston back into the bore. I started to turn it, it was extremely tight, and after a while it became easy and I made probably 100 rotations. But it was going nowhere at all. I tried turning the other way to see if it would move that way and the piston comes all the way out. So I finally got it back in and began turning unitl it got to a certain point and now it will no longer turn at all. The problem is it is about halfway or more out and there is no way it will fit over the rotor.

Any ideas?

Thanks

 

[MachAzure]

ttt

 

[MachAzure]

Anybody? bump

 

[n0v8or]

I used to have a Nissan 200SX with a similar rear caliper design. You had to rotate the piston to retract it for new pad installation. After a few years, I found I could not rotate the piston all the way in, so had to grind off some of the pad material to fit them in.
The cause turned out to be corrosion of the outer surface of the piston in the area where it protruded from the cylinder. Surface pitting prevented it from being pished back into the bore. Solution was to buy new calipers, always grease the piston seal when the pads are changed, and do not let the pads waer more than 50% before changing them (this prevents the piston from ever protruding very far).