How can I repair a leak in my anti-siphon spigot?

My outside hose pressure relief valve leaks when the water is on. It comes out near the top of the valve where it connects to the facet.

I have been unable to remove it, the set screw is shot after 18 years, I can turn it about 3/4 of a turn and that is it.

Any secrets for a quick fix without having to replace the facet.