A headers purpose, like a lintel is to support the weight of the brick above an open void like a window or door. The most common header compositions used is either precast concrete or stone. These units will fail over time due to structural settling or water penetration through the brick above. Unfortunately when they fail, there’s no way to restore them. Once the header is compromised, it can no longer bear the weight of the brick above. The only solution is replacement.
When removing and replacing load-bearing materials from historic structures, enormous research and preparation is taken to preserve the original design and functionality of the building.