There has been an increasing tendency to determine liquid metal quality using pressure filtration methods such as Prefil Footprinter tests. The scientific understanding of this method, however, is emerging only recently with the introduction of derivative methods. Based on the investigation of flow behavior over the pressure filtration test, classic filtration theory has been successfully applied to understand the filtrate weight versus filtration time curve. The “clean” and “effective” resistance of filter medium, overall filtration resistance, cake resistance, pressure loss and distribution can be quantitatively calculated now. In the present work, a typical weight versus time curve from the Prefil Footprinter test of a cast Al-Si alloy is analyzed and discussed in details. The method developed in the work can provide new insights into the filtration behavior.