Backwashing fluid filter assembly

A fluid pressure vessel has a filter cartridge with spaced headers positioning cylindrical pleated filter media. The headers form an inlet chamber communicating with the inlet upstream side of the filter media and an isolated outlet chamber communicating with the downstream side of the filter media. A hollow piston with a toothed periphery having a circumferential slot is positioned closely adjacent the upstream side of the filter media. The piston hollow communicates with a drain chamber isolated from the inlet and outlet chambers. Upon opening of the drain chamber to atmospheric pressure, the pressure in the piston hollow and slot causes a lower pressure in a localized area of the media and back flushing of trapped material on the upstream side of the media to the drain chamber. An actuator is connected to selectively progressively move the piston and slot along the entire surface of the media.