Weird. That's the one the parking brake uses, so that's the LAST caliper I'd expect to seize upI had the same problem as the OP on my 2011 Volt at around 23000 miles. Left rear brake caliper was corroded and inoperative. Had to replace it. I drive the Volt low mileage (it only has about 25000 miles on it now) and always in L, so I don't use the brakes much at all.