Replacing the piston and seal pack is super easy in any Clack water treatment valve.  Just follow the procedure in this blog post and video.  The procedure is the same whether you are working on a water softener, iron or sulphur filter or any back washable filter.

1) Start a regeneration and bypass the water softener. Disconnect the drain line and catch any water that might be in the drain line.

2) Remove the faceplate and unplug the power

3) Unplug the electrical connections and remove the Drive Bracket

4) Unscrew the Drive Cap Assembly and pull out the piston and regenerant piston.

5) Remove the Spacer stack assembly.

6) Once you have cleaned or replaced the required parts, reassemble in the reverse order.

The replacement seal pack and pistons are available here https://thewaterestore.com/search?type=product&q=clack*%20piston*