Unbolt caliper from bike.
Remove pads.
Use a c-clamp to slowly compress piston in caliper.
Check seals, and while in there clean and grease things.
The caliper piston should be able to be compressed with a c-clamp.
Not sure if the rear brake on our bikes has one pad that's stationary and one that moves, or if both compress the disc.
At any rate, I'd remove the caliper and inspect, clean, lube/grease, and see if that piston inside caliper moves or is stuck.
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2008 WR250X
Gearing: 13t - 48t
Power Commander 5 / PC-V
Airbox Door Removed - Flapper glued - AIS removed
FmF Q4
Bridgestone Battlax BT-003rs