1990 GMC Sierra 1500 Intermittent Drum Locking
 

I have a 1990 GMC Sierra 1500 step side with over 300K miles on it. Truck runs great with the recent exception of the rear drums intermittently locking up. Feels like I'm dragging an anchor just out of the blue. Back brakes do not release after braking master brake cylinder loose. When I release rear passenger side bleeder valve, brakes do release. Any ideas what to check next? Thanks!