There have been numerous cases of seized rear brakes here in the US. Some have been caused by faulty master cylinders and some by the caliper. I recommend starting with a good cleaning of the caliper as suggested by Murray. Then do a thorough bleeding of the system (with the caliper off the mount to allow the bleeder valve to face the correct direction). If you still experience brake problems it has to be the master cylinder. There are only two components in the system so process of elimination is in order. Best of luck!
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