Why would the rear brakes on a 95 Honda Accord stop working?

The car stops fine using just the front brakes, however the rear brakes are covered with rust. It was just at the Honda Dealership a couple months ago to replace the anti lock brake system sensors on two of the wheels and atleast one of them was a rear wheel. Would this be a caliper, brake line, master cylinder issue or just complete coincidence?|||There could be one of many things going on with your brakes. More than likely it could be the porportioning valve is working incorrectly, master cylinder has failed, or a break in lines. If there is no fluid leaking, then the likely cause would be the master cylinder. Also check your fluid level in your master cylinder's rear brake resevoir.|||Some cars have a pressure control valve that will send the fluid to only the rear or front is there is a leak for safety. However sometimes they can get stuck and only send fluid to one set of brakes even if their isn't a leak. There could also be a clog in one of the brake lines. Could be a problem with the master cylinder. Does the parking brake work the back calipers?