Rear brake pedal froze up.
Rear cylinder froze or ABS??
Happen to anybody??

Heat degradation is my guess. What happens is the buildup of this white crusty powder forms under the seal. Locks the piston from retracting back to square caused by the quad-seal's memory made of rubber.

Cut the circle of the seal and the end cut looks square box like. Water is heavier than oil so it settles at the lowest point, being the machined grove for the quad seal. Thus the memory to relax back to that square or its upright position.

As you push the piston out the quad rises up from the bottom like this [ // ]. The quad can pull the piston back to square like this [ || ] when pressure is released. But this crust buildup is locking piston in one direction and never pulls back off the pad.

Test to see if it's the master or caliper. Just snap the banjo bolt loose at the caliper. If wheel spins easy, it's the master. Wheel stays locked, it's the caliper quad needing a cleaning of that groove. Pull the quad ring out, seal up the banjo bolt hole, fill with vinegar and a little bit of hydrogen peroxide: let it sit overnight. Rinse with water and blow dry with compressed air.

Go outside and reflect the sun's rays into the groove to see if the grove is clean of the white buildup. Scrap with an ice pick, scribe, heavy gauge sewing needle. Spray away with contact cleaner and watch it evaporate in the sun. You can see leftovers where you missed. That or scrap before using the vinny and oxide.

Begin with the calipers. No sanding of any piston or caliper's machined inner surface. Wipe with a paper towel any liquid crud inside. Remember, vinegar expands any rubber.