On March 12, 2005, about 0920 Atlantic standard time, an Aero Commander 500, N8498C, registered to a private individual, collided with a ditch during the landing roll at the Fernando Luis Ribas Dominicci Airport, San Juan, Puerto Rico. Visual meteorological conditions prevailed at the time and a visual flight rules (VFR) flight plan was filed for the 14 CFR Part 91 personal flight from Fernando Luis Ribas Dominicci Airport, to Benjamin Rivera Noriega Airport, Isla de Culebra, Puerto Rico. The airplane was substantially damaged and the airline transport-rated pilot and three passengers were not injured. The flight originated about 7 minutes earlier from the Fernando Luis Ribas Dominicci Airport. The pilot stated that braking action was normal during taxiing, but after takeoff at approximately 1,000 feet above ground level, the main gear would not retract and he noticed the hydraulic pressure gauge indicating 0 pressure. He selected gear down, observed three green lights, used the hydraulic emergency hand operated pump, and turned back to the airport. While operating the hydraulic emergency hand pump, he noticed "some pressure" up to 800 psi. While en route to the departure airport, he advised the controller of the situation and moved the emergency hand pump selector valve to "brake only" to be able to stop the airplane after landing. He reportedly did not get any pressure while using the emergency hydraulic pump, and on final approach he slowed the airplane to approximately 85 miles-per-hour with the intention of landing at the very beginning of the runway. The airplane was landed, and he started pumping the emergency hydraulic pump for the brakes, with no success. Unable to stop the airplane, it collided with a ditch beyond the departure end of the runway. The airplane was moved to a facility on the airport and a mechanic stated to the FAA inspector-in-charge (FAA-IIC) that when the airplane arrived at his facility, the emergency hand pump selector valve was not set to "brakes only." The mechanic further stated that when he selected "brakes only", and operated the emergency hand pump, he obtained hydraulic pressure. According to the FAA-IIC, another mechanic performed maintenance to the airplane during the week of March 9, 2005. The work was performed to correct a hydraulic leak at the left main landing gear, and consisted of removal of the left main landing gear actuator to replace the fittings, and replacement of two flexible hydraulic hoses. The mechanic advised the FAA-IIC that the work was not completed and was pending a retraction test, and as such, he had not approved the airplane for return to service. NTSB review of the airplane's Maintenance Manual reveals that the emergency hand pump selector valve, located between the pilot and co-pilot seats, provides a means of directing the flow of hand pumped pressure to either the brake system or the normal hydraulic system. With the selector valve in "brakes only" position, only the wheel brake system and nose steering cylinder received hand pump pressure, which will not be indicated on the hydraulic pressure gauge.