An angle valve is the small brass fitting under every sink and behind every toilet. When it drips, your cabinet floor gets wet and mineral buildup eats at whatever it touches.
The good news: a leaking angle valve rarely needs replacing. Most leaks come from one of three spots, each fixable in under twenty minutes. The real task is figuring out where the water is coming from.
Wipe the valve dry and watch. Is water seeping from around the handle stem? That is a packing nut or O-ring problem. Coming from where the valve threads onto the wall pipe? The thread seal is failing. Dripping at the hose outlet? Gasket is shot.
For a handle leak, tighten the packing nut an eighth of a turn clockwise. Try the handle a few times, wipe it dry. Usually that stops it. If not, the stem O-ring has hardened. Shut off the main water, pull the handle, unscrew the packing nut, slide out the stem. Match the O-ring at a hardware store, pop the new one on with silicone grease, reassemble.
For a wall-side leak, shut off the main water, unscrew the valve. Clean both threads with a wire brush, wrap the pipe threads with PTFE tape three turns clockwise, screw the valve back hand-tight plus a quarter turn. Do not overtighten. Brass threads strip.
For a hose outlet leak, unscrew the supply hose, pull the old rubber gasket with pliers, drop in a new one, reconnect hand-tight plus a quarter turn.
The only time to replace: a hairline crack on the valve body. You will spot a white calcium line tracing the crack, usually from freezing or overtightening years ago. That cannot be fixed.
A brass angle valve machined from a proper casting rarely cracks. The weak points are cheap consumables: O-rings, gaskets, tape. Know where the water is coming from, and the fix is a five-dollar trip to the hardware store.