I could be completely wrong but it sounds to me like you are using the back brakes to stop the bike. The front brakes are where all of your stopping power is and the back are to assist the front. As far as the brake dust goes you are going to get a little. Like I said I could be completely wrong but it is hard to say without actually seeing it. Have you ever had the rear tire changed and if so was it put back together correctly? If the bracket for the rear caliper is not properly spaced it could rub and cause heat and premature brake wear. This is all speculation but it may give you a place to start.