Fluidmaster® recommends the PerforMAX® fill valve for toilets manufactured from 1994 to the current time (works on older toilets). If you are replacing the fill valve, we recommend you also replace the flapper with the 502 flapper for the ideal performance. The 502 flapper works on both old and new 2 in flush valve toilets. The most powerful toilet fill valve on the market. The 400AH is also known as the 400ARHR. With efficient and ultra-quite fill the 400AH replaces other inefficient or outdated fill valve designs. Help prevent excessive or inadequate bowl fill levels. The 400AH is a code approved, anti-siphon design, that won't contaminate the fresh water supply. Some toilet models not supported - Glacier Bay® or Niagara® flapperless toilets, one-piece toilets.