I have an old single-knob shower that uses a Grohe PBV cartridge. This cartridge is susceptible to being clogged with particles of grit from the plumbing, which reduces the water pressure. As a result, the cartridge has to be replaced periodically, which is expensive. (Replacing the shower isn't an option for various reasons.) I've found that the problem can be alleviated by removing the cartridge and picking grit out with a sewing needle. However, I don't understand how to reinstall the cartridge; when I try, the shower just runs constantly until I call a plumber to reinstall the cartridge correctly. The cartridge has to be positioned right, and I can't figure out the position. (I know I should have asked the plumber for a detailed explanation, but I stupidly forgot to do so.) For future reference, does anyone understand how the positioning of the cartridge works relative to turning the shower on and off, and how to properly position the cartridge when installing/reinstalling it?

If anyone knows, please post replies here or email me at david_b_mitchell_42@hotmail.com. Thanks.