On August 26, 2020, about 0815 Pacific daylight time, a Cessna 210, N9540T, was substantially damaged when it was involved in an accident near Chino, California. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that the accident flight was the first flight since the airplane had undergone landing gear maintenance. While in the airport traffic pattern, he extended the landing gear; however, he noticed that it would not lock into the down position despite multiple attempts to cycle the landing gear, which included using the emergency landing gear extension procedure. The pilot said that he by twisted the landing gear extension handle to the left to unlock, pulled the handle out, and pumped it vertically up and down about 50 times with zero resistance. The pilot was unable to obtain a positive down and locked indication. The pilot stated that during landing on runway 26R, both main landing gear collapsed, the left wing and left horizontal stabilizer impacted the runway surface, and the airplane came to rest upright on the runway. Postaccident examination of the airplane by a Federal Aviation Administration inspector revealed that the left horizontal stabilizer and elevator were substantially damaged. Subsequent examination of the landing gear revealed that the landing gear power pack fluid level was about 12 ounces low from full, and the engine-driven hydraulic pump was not operational. No issues were noted with the emergency gear extension system. The airplane was placed on jacks and a hydraulic mule was attached to the hydraulic system. The landing gear extended and locked in place normally. The landing gear was retracted, and the emergency landing gear extension system was operated, and the landing gear extended and locked into place. The engine-driven hydraulic pump was removed from the engine, and the crankshaft was rotated by hand using the propeller. No evidence of any mechanical damage was observed to the engine accessory case. A visual inspection of the hydraulic pump revealed a torn section of a seal that surrounded the pump drive shaft. Further examination of the engine-driven hydraulic pump at a repair facility revealed that the load spring was misshaped, and the oil seal was hard, brittle, and torn, which resulted in an oil leak and the pump unable to hold pressure.