A dishwasher has a wash chamber and sump disposed below the wash chamber, wherein wash fluid supplied to the wash chamber collects in the sump. A main pump is configured to drawn wash fluid from the sump and supply the wash fluid to the wash chamber, for example via spray arm assemblies. A filter is operably disposed to filter the wash fluid recirculated by the main pump. A backflush line has an outlet disposed downstream of the filter, and a backflush pump is configured to supply backflush fluid through the backflush line against the filter. A reservoir separate from the sump is provided and sized to hold a measured amount of fluid. The backflush pump draws the backflush fluid from the reservoir at a defined time.