ZRE152R - rear caliper not releasing properly

[the_random_hero]

G'day,

Vehicle is a ZRE152R Corolla which has done plenty of kms without too many issues. A while ago I did notice the rear right caliper dragging, and put it off as I couldn't find a rebuild kit. Eventually it got pretty bad, so over the weekend I've put on new rear pads/disc, as well as a wrecker caliper that was in very good condition. Noticed on the original disc that the back had worn significantly more than the front.

As part of this I pulled the caliper sliders out of the  original carrier bracket, cleaned/degreased the entire bracket, put on new lithium grease and installed with new seals. Slider pins move freely in and out and have next to no wear. 

Bled the brakes, bedded the new pads in and took it for a quick drive - everything seemed fine but I thought I could hear a little bit of dragging still. Didn't think too much of it until I got home and noticed that the inner pad was cooked - it had been binding on the side closest to the center of the hub with wearing about half a mm into the new disc already.

Since then I've pulled the wheel off, cleaned/regreased everything and checked everything functions as it should. I disconnected the handbrake at the caliper end, and pressing on the brake everything works well - but it doesn't release fully. I can wind the piston in without an issue, and it will extend without the pads in fine. 

Wracking my brains trying to think what else the issue could be - spoken to a few people who are insistent it's the slider pins binding up, but I've confirmed this isn't the case.

Cheers