Abstract: A pressure compensating lubricant reservoir system is disclosed for a rotary cone rock bit. The system comprises a two-piece canister and flexible boot, one side of the boot being vented to the dome area of the bit. An O-ring is utilized adjacent the reservoir cover cap to seal the lubricant within the reservoir cavity. In order to prevent damage to the O-ring, the cover cap part of the canister forms a circumferential groove therearound to accept the O-ring. An intermediate disc is trapped between the cover cap and the resilient boot and serves to close out the O-ring groove in the cover cap.With a prior art one-piece canister, the O-ring is put into position in a groove formed by the canister, followed by insertion of the canister into a lubricant cavity formed by the bit body. Damage to the O-ring often occurs when the outer peripheral edge of the O-ring passes by a cover cap snap ring retention slot at the reservoir cavity entrance.