Have you tried adjusting the front brakes and bleeding them?
If you haven't already, you'll probably have to take the cover off the back brakes and take the drum off and clean it and the brakes shoes and maybe replace the shoes. The back brakes are bad about getting full of mud. You might want to take the little shaft that the brake lever clamps on, out and clean and put a thin coat of grease on it, so it will move easier.
If you haven't already, you'll probably have to take the cover off the back brakes and take the drum off and clean it and the brakes shoes and maybe replace the shoes. The back brakes are bad about getting full of mud. You might want to take the little shaft that the brake lever clamps on, out and clean and put a thin coat of grease on it, so it will move easier.