A vacuum breaker is designed to prevent water that leaves the faucet from flowing back into the plumbing system, in other words, vacuum breakers prevent water from flowing in the reverse direction. Any place where water is supplied for any use other than drinking water, needs a vacuum breaker attached to it.  Laws require vacuum breakers be installed on outdoor hydrants, commercial dishwashing machines, mop-sink or service sink faucets, and other commercial kitchen applications.
VMS stocks vacuum breakers and repair kits for American Standard, Chicago, Kohler, Speakman, T&S Brass, Zurn and others.