Replacement piston assembly for WaterSoft Isobar 1 filter control valves manufactured prior to 2010.  If you aren't sure how old yours is, take off the back cover and look at the color of the piston cap (also called the "end plug").  If your filter has an orange cap like this one, then you need to use this piston.

If your piston has a White Cap, you need the 20561X254

If your piston has a Black Cap, you need the 20562X254

 Note: We recommend installing a new seal & spacer kit anytime a piston is being replaced.