This paper describes the development of a new improved hydraulic O-ring seal material to replace the currently used -65°F to 275°F MIL-P-25732 specification hydraulic seal material. Test data at 275°F shows the MIL-P-25732 material has about a 500 hours operational life. This new material (AFE XN 1925-33) has at least double the life of the specification material. Also discussed is the performance of the retainer ring (backup ring) and recommendations for its use. The development of the O-ring concept, early performance history and the evolution of the O-ring to the most recent specification material (MIL-H-83461) are related.