1973 Dodge 200 truck

truck is rarely used, and now the brake pedal sticks and eingages brakes . I have to lift pedal back up to release brakes. I found one fluid chamber empty,assuming a leak and replaced with new master cyclinder, bleed and still the same problem. Is there a spring or something worn on the pedal? The brakes are power with disc in the front. Once I pull the pedal up manually brakes release.