caliper dragging on KZ750 twin

We thought the piston was sticking...but it's not. We popped it out and it's clean, the seals are pretty, and the passages are clear. But the piston doesn't push back and it drags the rotor.

The master cylinder isn't new, but I pulled it directly from another bike (swapped out to a newer radial master) who's brakes were working beautifully.

Any ideas?

Open the bleeder, or crack the line to see if there is any pressure left. Shouldn't be any. Possible return port in Master blocked.

No pressure on caliper without the lever being drawn...it's bleeding back properly.

Caliper slides, where it mounts with 2 bolts, free/ lubed? It should slid back and forth. Try just that without pads. Just don't pull the brake lever. Maybe push the piston back again.
It is the Square cut O-ring, that will move the piston back, just a bit, when pressure is released. Did you replace any of the caliper seals?

Motor Head wrote: Caliper slides, where it mounts with 2 bolts, free/ lubed?

That's what was hanging up on mine. I removed each mount bolt (one at a time) and greased and re-inserted. Afterwards the caliper slid easily on the pins.