Priming Buttons ((2) for Berkey water system Black Berkey Elements & PF2 Filters This little priming “button”, as it’s been called, allows you to prime your Black Berkey™ Elements (filters) using existing in-line plumbing. This priming ring can also be used to prime & back-flush the optional PF2 (Fluoride & Arsenic Reduction) Post-Filters. To prime the Black Berkey™ Elements when you don’t have access to plumbing, use the Black Berkey™ Primer.