I HATE Anti-siphon valves!
For those of you who don't have them, consider yourself lucky!
This house was built before the anti-siphon valve was required by most plumbing codes on outside hose faucets. I needed a new long-necked anti-freeze valve some years ago, and all of them were anti-siphon. So I bought one by American Valve.
First, some terminology... the part that sticks up outside is the VACUUM BREAKER valve. It is a one-way valve that keeps water IN when the valve is ON, and when the valve is turned OFF, it opens up under any vacuum condition to allow air in (to help break any siphon effect, like putting the working end of the hose in a pool).
More dastardly, is the Anti back-flow Check Valve that is inside the faucet body, way to the supply end. In a regular (now obsolete) freeze-proof faucet, the long shaft you turn via the handle screwed a rubber compression washer in and out, exposing/covering a hole in the casting. The washer was held onto the end of the shaft via a brass screw in the center. To change the washer, because the valve would not shut off all the way unless to murdered it closed, you turn off the house water, and unscrew the packing nut (the shaft goes through it) with a wrench, while holding the faucet body from turning, with another wrench.
However, the new valves implement a spring-loaded valve idea back there. Where the original way was to attach the rubber washer directly onto the end of the valve shaft, the washer is now attached to a spring-loaded cup, such that the cup with attached washer can move in/out against spring pressure when the bib is ON. So when you turn the valve ON, the long shaft backs out, but the spring keeps the washer over the hole, keeping the water off... but water main pressure pushes the spring-loaded washer outwards, allowing water to flow. If city water pressure is lost while the hose bib is ON, the spring closes it, which creates the anti-backflow feature.
Sounds good, right? Uh, noooo... that spring-loaded washer is a real pain. It likes to oscillate open/closed like a motor boat under certain and extremely common conditions. An impulse sprinkler drives it nuts. Often, when using the hose outside, if someone inside turns on a faucet near the hose bib, the whole house cold water system can hammer away in resonance. Also, the spring-loaded washer can stick when not used frequently, creating the odd effect of turning the valve knob ON, and no water flows, then suddenly does. Can be very hard to regulate flow.
I would have put my old valve back in, but its body was worn out, the long valve shaft wobbled too much on it.