Frost free yard hydrants are great in places where the winter gets cold enough to freeze water. Usually yard hydrants work well for many years. When a hydrant begins to leak or drip it can be a combination of age and temperature changes that finally wear down the rubber gaskets.
To stop the leak the first thing to try is a yard hydrant repair that involves replacing the gasket. If that doesn't work or if by chance the riser that holds the gasket breaks you can simply replace the hydrant all together.