On July 29, 2005, about 1430 Pacific daylight time, the private pilot flying a Beech B23, N6121N, experienced a loss of directional control during landing rollout on runway 18 at the Fallbrook Community Airpark, Fallbrook, California. The airplane veered left, exited off the side of the runway, and impacted an unoccupied Cessna 180 that was parked on the tarmac. The Beech was substantially damaged. The pilot, who owned and operated the airplane, sustained a minor injury. Visual meteorological conditions prevailed at the time of the personal flight, and no flight plan had been filed. The flight was performed under the provisions of 14 CFR Part 91, and it originated from Grass Valley, California, about 1030. The pilot reported to the National Transportation Safety Board investigator that his approach to the airport was normal, and the airplane touched down without mishap near the centerline of the runway. The wind was from 180 degrees, at 8 knots. In the pilot's completed "Aircraft Accident Report" he indicated that the airplane swerved sharply to the left immediately after he applied right and left brakes. Repeated application of right brake appeared to have no effect on straightening the airplane, which continued veering left until exiting the runway. The pilot stated to the Safety Board investigator that his airplane's left brake seemed to partially lock up during the landing rollout, and thereafter he was not able to maintain directional control. Responding airport personnel reported observing evidence of both left and right wheel tire tread (skid marks) on the tarmac leading from the runway to the accident airplane. Under the Safety Board investigator's direction, Federal Aviation Administration (FAA) personnel and a contract FAA certificated mechanic examined the airplane. In summary, both wheels were found to turn freely and brake application produced normal results. No evidence of any mechanical malfunction with the airplane's brakes, wheels, or tires was found.