I need to replace the cartridge in my shower valve. It is an Adia which I believe was a line of Dornbracht valves. It is a single handle model and probable at least 20 years old. The knob is not screwed on but simply pulls off. It appears that the end of the cartridge is plastic with teeth that mesh with the knob. I see no way to remove the plastic piece. I don't see how to pull the cartridge without damaging the plastic piece. I've look ****** and at Dornbracht's web site to no avial. Any ideas?