That Dripping Tap

Every time that I have had to replace a tap washer I have always found that the seating surface within the tap body has this annoying eroded leakage path approx 1mm wide necessitating a time consuming re-seating of the valve surface! In this shot you can see the eroded mouth at the inner diameter- the erosive power of water. The copper appearance of the threaded portion is suggestive of de-zincification of the brass - perhaps the tap was cast from a low quality brass!