your cylinder, return spring, or parking brake cable is sticking, make sure the cable hasn't rusted up
right now rock auto has a bunch of brake parts on clearance, you can get the new rear cylenders for like 6 bucks,
i would recomend this kit too
http://www.rockauto.com/catalog/rafr...&parttype=1772
http://www.rockauto.com/catalog/rafr...&parttype=1892
these too.
if your doing it all i say go all out, if you can get a good price on front calipers get those too,
bad thing with rockauto it cost alot to ship these heavy rotors and drums