a few rides ago one of the pads wore right down but I was still using it hard, so much so that one of the pistons popped out a tiny bit & tilted to the side. I pushed it back but on 2 the rides I have done on that bike since there has been a horrible scraping noise, like when your pad is down to metal. I figure piston isn’t returning properly as the pads are brand new. Remedy? drain it, take it apart, clean it all, reassemble and fill up with oil & bleed?
it is a shimano deore. any thoughts?

YOu might be lucky and get away with a careful rebuild but you might have damaged the seal. I don’t know what the scraping noise is – is the spring touching the disc?

Have you re-alligned the brakes for the new pad? Are the brakes working (ie stopping you)? Is there fresh contaminent on your new pads now? Like other poster said think the seal may be gone, but you might be lucky

any idea where I can buy a new seal?

You can’t. calipers are cheap tho

Did you lose the brake fluid?

Pistons are an extremely tight fit in their seals and I’d guess it’s difficult to damage the seals. What happened when the pad wore right down? Were you running metal to metal? Are you sure it didn’t score the rotor?

If you do have a sticky piston then it should be easy to free up. Simply remove the wheel and hold back the piston that freely moves. Then apply the brake lever. This will force the sticky piston to move.Push it back in and repeat.
This should free it up