An arrangement and method for maintaining a minimum flow velocity in return piping receiving dirty coolant from a plurality of machine tools receiving filtered coolant from a filter apparatus. A bypass line diverts clean coolant into the return piping via a pressure reducing valve set to allow make up flow of clean coolant into the return piping just sufficient to maintain a pressure corresponding to the minimum flow velocity in the return piping. A pressure maintaining valve is interposed upstream of the pressure reducing valve and set to restrict flow as necessary to maintain adequate pressure to insure that the machine tools are adequately supplied with clean coolant at start up prior to the beginning of contaminated flow in the return piping.