9" rears dragging

In my opinion it is definitely in the master brake cylinder or wheel cylinders. Step on the brakes and the pressure is on and brakes won't release. Leave them alone for awhile and they slowly release on their own. Just like a slow draining sink.
I had the same problem with my wheel cylinders. They were filled with many years of old gunk. The gunk was plugging them up and they would lock up and then slowly release.
You have new brake cylinders so I would think the master cylinder is the culprit.