It is a very simple job. Open the rear brake fluid reservoir, and use a plastic or hardwood tool to push the pistons back into the caliper. Once this is done you can remove the caliper retainer bolts. The caliper will come right off and the pads will fall into your hands. Since you are worn to the metal a new rotor is in order, The muffler, and rear wheel have to be removed for this. I ALWAYS lube the Caliper mounting bolts w aluminum anti -sieze compound before re-assembly. You should also change and bleed the brake fliuid, because of the heat build up from the worn pads. Even w/ the rotor replacement the whole job should be around 2 hours.
