A Brian Cache is a huge storage cache of Clan military hardware.
Early in Clan history, General Aleksandr Kerensky ordered a controlled demobilization of 75 percent of the Star League Defense Force that accompanied him on his Exodus to provide the needed civilian support population. This resulted in a surplus of military hardware that was stored, heavily fortified, and guarded by loyal troops.  
During the Pentagon Civil War, many rebel factions stormed and looted Brian Caches in the Pentagon Worlds. Some Caches were simply overrun while rogue SLDF soldiers looted some others themselves to form their own factions. After Operation Klondike, the remaining Caches (and the considerable number in the Kerensky Cluster) were distributed among the Clans according to their socialist principles.
Brian Caches have been the catalyst for conflict between rival Clans, as possession of one nets the Clan considerable war material and other resources. One such Brian Cache at Circe, containing the equipment of the former 331st Royal BattleMech Division, was one of the catalysts for the Wolverine Annihilation.
During the Clans' Golden Century and the years prior to Operation REVIVAL, the Brian Caches were further enlarged when "obsolete" Clan equipment were placed into mothballs after newer-generation equipment became available. The aftermath of the Great Refusal and the Wars of Possession in the Clan Homeworlds saw many Clans reactivate some of their Brian Caches to update outdated equipment with cutting-edge technology. These were then used to equip more forces to fight their Trials. Examples include Clan Star Adder's Hellfire (from the Lupus) and the Steel Viper Scylla (from the Storm Giant). The Wolves emptied their remaining Brian Caches to bolster their severely-weakened Touman following the Refusal War.
The Brian Caches would play their final and most prominent role during the Wars of Reaving. During the Society's uprising, Brian Caches were subverted from within or without by Society-aligned forces or their Dark Caste allies to quickly equip their meagre forces. As the Society's scientists knew of all the codes and procedures for opening their respective Clan's Brian Caches, it was a simple matter for them to take control of these facilities during the chaos and turmoil of the opening phases of the Wars.  By the end of the conflict in early 3076, presumably all remaining Brian Caches had been exhausted of their possessions. The surviving Homeworld Clans would have to replenish their depleted Toumans through new production runs, which coupled with the severely-diminished post-war output, was one of the main factors in their slow recovery.
- Field Manual: Crusader Clans, p. 159 "Glossary"
- Field Manual: Warden Clans, p. 168 "Glossary"
- The Wars of Reaving, p. 92 - "Equipping the Invisible"