the inlet valve 20 is opened as illustrated in fig4 but when the inlet valve 20 is fully closed the backwash valve 26 is opened as illustrated in fig5 as illustrated in fig1 under 34 normal 34 operation the filter unit 12 of the invention 10 can filter a liquid fluid by passing the fluid through the inlet valve 20 through the backwash valve body 58 through the filter cartridge 42 and out through the outlet 18 via the outlet valve 22 material filtered out of the fluid is trapped within the filter cartridge 42 during normal operation a reservoir of filtered fluid is trapped within the reservoir 38 the fluid within the reservoir 38 is pressurized by air pressure trapped at the top of the reservoir 38 during normal filtering operation such as illustrated in fig1 the backwash valve 26 is maintained tightly sealed against the valve seat 60 of the backwash within they fluid pressure within the backwash valve body 58 as illustrated in fig2 during backwashing operation both the inlet valve 20 and the outlet valves 22 are closed and the backwash valve 26 is opened in this configuration pressurized fluid within the reservoir 38 is caused to flow backwards through the filter cartridge 42 to dislodge filtered material from the filter cartridge 42 and flush such material out of the filter unit 12 through the backwash valve 26 to the discharge zone the backwashing operation is initiated by actuator means which comprises an actuation member 68 and interconnecting means for interconnecting the actuation member 68 and the backwash valve member 62 in the embodiment illustrated in the drawings.