I removed the master cylinder and it was pretty disgusting so I cleaned it out thoroughly using brake fluid. There aren't any rebuild kits or even used master cylinders anywhere on the internet or CL so cleaning is the best I can do.
When I remounted the cylinder I made sure to spray down the lever components again with WD40 to make sure that they don't stick. I bled the brake and I found that the brake was still dragging. The lever rebounds fully and quickly, but the brake piston is still dragging on the rotor.
I removed the caliper, pushed the piston back in, then re-bled the brake to make sure it was okay. Again the brake was dragging.
I assume this means I need a caliper rebuild kit?
Current: '19 Harley Roadster, '72 XLCH, '84 GPz 550
Past: '84 GPz 550, '82 KZ750 LTD, '71 XLCH