Abstract: A downhole bearing assembly includes concentric inner and outer tubes, with bearings mounted therebetween. A high pressure elastomeric cup seal assembly is mounted between the inner and outer tubes, and at least one metal-to-metal seal is mounted between the inner and outer tubes on each side of the high pressure seal assembly. A pair of pressure balancing systems are provided, each acting to minimize pressure differentials across a respective one of the metal-to-metal seals. The metal-to-metal seals protect the high pressure elastomeric seal assembly and the bearings from contamination by drilling mud, and the pressure balancing systems protect the metal-to-metal seals from excessive differential pressures.