Brake Drum Wheel Cylinder problems of the 2011 Chevrolet Silverado 1500

My problem started around October 25, 2014, a week before I took it to the dealership. . The back brakes started to chatter when I applied the brakes. It was coming from the back brakes not the front. . The dealership told me the inspection showed the lr wheel cylinder leaking heavily and contamination shoes. Rr also seeping. Nec to replace rear wheel cylinders and bleed system. Replaced rear shoes and cleaned affected areas. Resurfaced drums and reassembled. I paid $514. 74 for this, and with a reward card, I got $15. 00 off, bringing the total to $499. 74. I talked to the service mgr. From hamilton Chevrolet on 11/7/14, and he said he would see if he could get me a refund for half of the charge, and should get the check from gm in about two wks. I feel that this is a gm problem, and with only 35, 574 miles on my truck this should not have happened. I am a 73 yr. Old retire, and having to pay for this repair really set me back. I have owned many gm vehicles, and have never had any problems. I take care of my vehicles, and make sure they are serviced regularly. I would appreciate it if you could look into this, and get back to me. Thank you.