Most anti-siphon faucets can be repaired with a simple cleaning. Pry off the top cap with a screwdriver. Then unscrew the plunger assembly. Clean all the parts thoroughly and replace any that are worn out. Put the assembly back together and snap the top cap back in place. This should fix any leaks you had. More information about anti-siphon faucets can be found here
. http://360homestour.com/anti-siphon-spigots.htm.