The airplane had been out of service for six months due to leaking hydraulic sequence valves. Following installation of the repaired components, maintenance personnel cycled the landing gear multiple times through its full range of travel. System pressure was maintained and no leaks were observed. The airplane was subsequently serviced with fuel, oil, and hydraulic fluid in preparation for a maintenance test flight. The pilot reported that a preflight inspection was completed, and the three crew members boarded the airplane for departure. The before takeoff checks, including a full power run-up of both engines, were completed with no anomalies noted. After takeoff and following the selection of the landing gear to the up position, at about 300 ft above ground level (agl), the flight engineer noticed they were losing hydraulic fluid from the right main landing gear. The left main and nose landing gear had fully retracted; however, the right main landing gear was partially retracted and not in its wheel well. The auxiliary hydraulic pump was selected on, followed by the selection of the landing gear to the down position. The left main gear subsequently indicated down and locked with the green gear position light illuminated; however, the nose gear was in the up position, and the right main landing gear was down but not locked. The flight crew climbed the airplane to about 1,500 ft agl and circled the airport, while the emergency landing gear extension procedure was performed. Following completion of the procedure, the three landing gear green position lights were illuminated indicating that the landing gear was down and locked, and the hydraulic system indicated no system pressure. The flight crew conducted a low pass over the airport while communicating with ground personnel who visually confirmed that three landing gear were in the down position. The airplane returned for landing, and the before landing checklist was completed. Upon touchdown, about 1,500 ft down the runway, the left main landing gear slowly retracted, and the airplane began to veer left. The pilot applied right brake and full right rudder in an attempt to correct for the veer, to no avail. The airplane departed the left side of the runway, the left wing struck a sign before impacting the ground, which resulted in substantial damage to the left wing and hull. Postaccident examination of the airplane revealed that all three green landing gear position lights were illuminated; however, the left main landing gear had collapsed. Further examination revealed that the left main landing gear hydraulic actuator attachment bolt had sheared at the lower actuator attach point. In addition, the right wheel well was coated in hydraulic fluid, and the leak appeared to be from a hydraulic line that ran from the bottom of the actuator to the sequence valve. Examination of the failed landing gear bolt revealed reverse bending fatigue resulting from bending loads. The fractured surface exhibited a distinct overstress region with beach marks visible on both sides.