I have been using one of these for about 7 years..... Not even one issue. Highly recomended!
http://www.marinedepot.com/Kent_Marine_Float_Valve_Kit_For_RO_and_RO_DI_Units_Float_Valves_Controllers_for_RO_DI_Water_Filter_Systems-Kent_Marine-KM1651-FIRORAFV-2-vi.html