Ben,
You may wish to back wash your 4040 menbrane before you make another purchase. You simply connect your water supply to the permeate output of your R/O housing. Dont forget to disconnect your pre filters first!. According to GE, the manufacturers of the merlin, its life expectancy @ 600ltrs per day in avarage TDS (250TDS) water is only 6 months so it maybe better to replace your R/O membrane if it really is shot.
Regards
Carl.