The International CBX Owners Association

Yes, that's what happened to my bike and causing dragging. The piston seal had some black rubber bits from the brake hose embedded in it.
This video is a little clearer https://www.youtube.com/watch?v=KJ0koDXk6PY
Running at high speed would heat up the rotor, so I had to let it cool for 20 mins to release the piston.
I also was having some metal to metal contact on the rotor so bought a new one from eBay - Approximately US $108.92
https://www.ebay.com/itm/114768839733

The tiny compensating port in the master cylinder was also plugged, preventing brake fluid from going back into the reservoir.

The intake port or vent allows fluid to enter the rear of the cylinder, as the piston moves forward. Fluid flows out of the reservoir, through the intake port, and into the area behind the piston and cup. The compensating port releases extra pressure when the piston returns to the released position.


Brakes - Operation of the Master Cylinder from ST1300 forum
5 - Blocked Pressure Relief Port
https://www.st-owners.com/forums/thread ... er.159167/


Hopefully, my rear brakes are OK now.

Should be good to go now!