Frost proof faucet leak

I've got ongoing problems with a customer's frost proof anti-syphon hose bib installed in a garage that leaks from the syphon vent. I've installed three faucets and they all leak to some extent at the vent. Unfortunately, thanks to the governmental agencies, you cannot find an exterior faucet without this feature anymore. Any help? The faucet leaks about 5 gal/day. Can the anti-syphon be adjusted or plugged up?

Are you leaving the faucet on and trying to stop the flow with a spray handle on the hose? This is not how they're designed. Any pressure hiccup will dump water out the anti-siphon. It if is leaking when it is turned off, you've got a defective valve(s).