Two days ago I didn't know what they were and had never seen one before (thanks to folks here for enlightening me). So today I installed my first vacuum breaker (aka anti-siphon device) on a customer's hose bib.
It was something of a bitch. I noticed the setscrew on the new one I bought, but couldn't see or feel any such screw on the old one, just a round bump. So I just torqued the shit out of it with a pipe wrench (crescent wrench holding the valve body). I threaded the new breaker on, then tightened the setscrew. After just a few turns, it promptly broke off, apparently just as it was designed to do (I could see it had a narrow shank).
So what's the deal with these? Are you expected to replace the hose bib and vacuum breaker as a unit when either one fails? After breaking off the setscrew, there's no way in hell to get it out. (Wrenching it off did chew up the threads some, but there was enough left to secure hold the new breaker.)
The man at my plumbing supply place said he understood that inspectors in Berkeley and Oakland were requiring these on new residential construction (they're been required for commercial sites for some time now).