A filter including a first chamber; a second chamber; a piston having a head portion disposed in the first chamber and a rod portion disposed in the second chamber; a third chamber containing a filter medium, having an inlet communicating with the second chamber and an outlet; first and second conduits supplying fluid under pressure to the first and second chambers, respectively; and a valve in the first conduit operable to direct fluid under pressure to opposite sides of the piston head portion to reciprocate the piston and correspondingly eject fluid in the second chamber through the third chamber.