Yep a piston is stuck. Throw on a set of rebuilt calipers, and new hoses, flush and make sure to tighten the heck out of the caliper bolts.

While your back there yank off the disc if it hasn't been off in awhile and check/lube/clean the parking brakes. A lot of times the shoes are gone and need replacing.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200