Hydrodynamic regime of impregnation of the dispersed filler with melt by the vacuum intake under isothermal conditions is investigated theoretically and experimentally. An influence of oxide film on permeation pressure of aluminum alloy (AMg5K) melt in the capillary clearances of filler is studied. It is shown that two stages of impregnating process take place. A reason for a flaw of porous castings in uniformity of properties that is unavoidable for given technological process is revealed.