Brush seals promise improvements to the widely used labyrinth seal in regulating turbomachinery leakages. Enhanced resistance to the flow is provided by a static ring of densely packed fine wire bristles that are angled in the direction of rotation and flex to accommodate rotor excursions. A large-scale brush seal was constructed to study the leakage characteristics in direct relation to the pressure field within and surrounding the bristle pack for multiple clearance conditions, therefore developing the understanding of brush seal fluid dynamic behavior. The governing parameter controlling leakage behavior transitioned from pressure ratio for a large clearance, to pressure load for a line-on-line configuration. In all cases, leakage flow converged to an asymptotic value once maximum levels of bristle blow-down and pack compaction were attained. For both clearance configurations, this occurred at a pressure ratio corresponding to that at which axial distributions of pressure converged; equivalent behavior was noted for the line-on-line configuration with pressure drop. Comparatively small changes were experienced in leakage behavior and in the interbristle pressure field with increasing pressure drop for the line-on-line brush seal. This indicated that brush seal performance is more influenced by changes in bristle blow-down than bristle pack compaction.