Just went though the same problem. Sounds like you have contaminated brake shoe linings.Did you happen to get dirty hands on shoe linings or drums (grease,oil or dirt ect.)? If you have bled all air from system and pre adjusted brakes where the pedal is hard and the emergency brake handle feels like it has correct resistance there is no thing else it can be. If all this has been performed pull the brake adjuster grumets out of backing plate and blow a little baby powder in the brakes and test. An old timer had taught me this trick and has worked for me in the same situation. I know it sounds crazy but baby powder has talcum in it wich will absorb the contaminates. hopefully this will help you out.