you may have glazed the pad (or rotor) from light braking or improper break in. Get up to about 50 or so and apply the rear break hard but CAREFULLY. do not stomp it down but put nice, smooth firm pressure on it making sure you do not lock it up. Do this while holding the throttle on so you maintain your basic speed. Do this for about 3-5 seconds and repeat a few times. if it does not stop the squeal after a couple of attempts, try something else. By the way, a good way to stop the rear from making noise is not to use it. Fronts have all your stopping power- back is good for holding you still at a stoplight and parking in the garage.