Probably rusted, if your handy, you'll have to get under the car to check the cables and where it connects at the rear brakes. Also take the rear wheel off and take off the drums to inspect. otherwise have a local garage inspect it for you.
Might get lucky releasing it backing up. But I will add if its sticking you likely will need rear brakes soon and need to get that done too. May even be best to get a rebuild kit for each rear, as likely needs rebuilt at its age, not just brake shoes and while there wheel cylinders too.
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