stops fluid continues to be pumped through filter 202 a and on to the disposal area during pump cycle 1 at the end of pump cycle 1 fluid flow through open check valve 205 a stops and hence no pressure is applied to pressure controlled drain valve 316 of flush valve 204 a additionally the fluid flow through cross feed tube 228 to flush valve 204 b also stops consequently without pressure being applied pressure controlled drain valves 316 of both flush valves 204 a and 204 b open and the fluid within the respective conduits 308 drains back into holding tank 110 the corresponding buoyant plastic balls 305 fall and flush valves 204 a and 204 b are ready for pump cycle 2 pump cycle 2 as shown in fig4 b proceeds similar to pump cycle 1 shown in fig4 a in this case filter 202 b is providing fluid to the disposal area while filter 202 a is being back flushed under the control of flush valve 204 a in an alternate embodiment a ball valve or similar valve can be disposed within the fluid path of cross feed 228 of fig2 a to control the back flushing duration this optional valve 401 is shown in broken lines in fig4 a and 4b in particular to decrease the rate of back flushing valve 401 is opened to increase the rate of flow into the given flush valve 204 with the shortest back flushing period resulting when valve 401 is fully open on the other hand to increase the rate of back flushing valve 401 is closed to decrease the flow into the given flush valve 204 although the invention has been described with reference to specific embodiments these descriptions are not meant.