Why is my outdoor spigot vacuum breaker still spraying after I replaced it?

The fact that you have a black hose with an extra shut-off valve attached is the biggest clue here. Most of these vacuum breakers on frost-proof hydrants aren't designed to handle **constant pressure**. If you're leaving the main spigot handle turned on and using that secondary valve at the end of the hose to control the water, you're trapping backpressure against a component that's only meant for intermittent flow. By code, these are usally only rated for about 12 hours of pressure in a 24-hour period. If it's pressurized 24/7, the diaphragm is going to fail or leak almost every time. You also need to take a close look at the plastic seat where that rubber plunger sits inside the breaker housing. Even a tiny grain of sand or a hairline crack in the brass housing will prevent a seal, no matter how many kits you buy. Since you mentioned tightening it down hard last year, there's a real possibility the threads or the seat got slightly deformed. If the housing is warped, the new rubber seal won't sit flush against the surface. Also, make sure you got the exact OEM replacement. These things look generic, but brands like Woodford, Prier, and Arrowhead all have slightly diferent tolerances. A "universal" kit from a big-box store often doesn't fit quite right. Find the brand name on the faucet cap or the handle and order the specific repair kit for that model. If you can't find a name, take the old parts to an actual plumbing supply house rather than a general hardware store; they can usually identify the manufacturer on sight. As for capping it off, you relly shouldn't. That breaker is there to stop dirty hose water from siphoning back into your home's drinking water if the city pressure drops. If a new OEM kit doesn't fix it, the internal seat is probaly shot. Instead of tearing out the whole wall imediately, see if you can just replace the internal stem first. But if you do have to swap the whole fixture, just install an **access panel** in the basement drywall afterward. It makes the next repair a five-minute job instead of a major renovation.

I had no idea about that 12-hour pressure rule. I've been leaving my hose pressurized all summer to run a timer for my garden. Does that mean those battery-powered hose timers basically kill these vacuum breakers eventually?

Also, if the threads are actually warped like you mentioned, is there any kind of waterproof grease or somethig that can help it seat better, or is it just game over once the brass is bent? I'm terrified of having to cut into my basement ceiling just for a leaky spigot.

Forget the grease. If the brass is warped or the seat is gouged, grease just gets washed away. Those battery timers are definitely "breaker killers" because they keep the hydrant pressurized 24/7, wich keeps the diaphragm constantly stressed.

Check the top rim of the housing for burrs or nicks. Sometimes a bit of light sanding with high-grit emery cloth can smooth out a rough seat enough to get a seal. But if you've tried an OEM kit and it's still spraying, the housing itself likely has a hairline crack from over-tightening. If that brass is deformed, there realy isn't a way to save it.

Check the packing nut behind the handle while you're at it. Sometimes water leaks from the stem, hits the back of that recessed box, and sprays everywhere, making it look like the vacuum breaker is the only problem.

Also, double-check that you actually removed the old plastic seat if the kit came with one. Sometimes the old one gets stuck in there and you end up stacking the new parts on top of the old junk, wich guarantees a leak. If you see two layers of plastic or the cap won't thread down all the way, that's your issue.