The pilot made a normal landing on runway 16 in his amateur built experimental airplane. The pilot reported that his airplane's left brake became dysfunctional during rollout, and he was unable to stop before reaching the runway's departure end. The airplane rolled over the runway's end, entered a ditch and nosed over. The airplane had decelerated to about 10 miles per hour when it exited the runway. An examination of the airplane's brake system revealed that the left side master brake cylinder had failed.