The piston for my wall-mounted Simplehuman soap dispenser broke after 3 years of use. It's translucent plastic. You get to it by removing the allen head screws (x2) under the caps on the side of the dispenser. Then remove the 3 screws underneath the steel cover and the 4 screws on the mounting pegs on the back, then pull it apart. The pump will just pop out then remove the round black cap by pulling on it with your fingers and then take pull apart the top and bottom and the piston will pop out.

Rather than toss it I whipped up this piston. The only way I could get it to seal properly was by adding an o-ring around the piston. Soap dispenser is back up and running. Hope it helps someone else out with the same problem.