How can you tell when a vacuum breaker needs to be replaced?
In order to answer the question with any assurance I would have to know what you mean by "leaking."  However I am willing to hazard the guess that the vacuum breaker is indeed operating as designed - that is to say it introduces air into the pipe/hose when a vacuum is induced.  This would cause the water to flow, and water flows in whichever direction is easiest.