Abstract: In combination with extrusion apparatus having a liquid source, a die and alternate filter means, a continuous flow plug valve unit is provided for use in changing filters; the valve unit comprises a valve housing, plug means seated in the housing, and control means for moving the plug means with respect to the housing. Passage means in the plug means and housing overlap in all operative positions of the valve to provide an available flow passage of hydraulic flow radius everywhere at least about equal to that of the inlet from the source; the valve presents no restriction to the flow of liquid from the source.