Frost-free hydrants can be a real pain. If you don't get suction from the outlet when you turn it off, there will still be water in the riser and it will freeze overnight.
I fought one of these rascals for a year and a half before I solved it.
I replaced all the washers in the valve. I knew the bottom had drainage because I had but it in some years ago. One day, I noticed it was dripping out of the outlet even though it was turned off.
Finally dug the damn thing up - it was buried about six feet deep. Got a brand new hydrant and hooked it up. Just thought I might check it before I filled the hole back up. Lo and behold, there was water rising in the bottom of the hole. Further digging and sucking the water out with a shop vacuum disclosed a leak in the supply pipe where it came out of the well.
When I put it all back, I put a 3 inch pvc sleeve about the hydrant so I can easily change it if need be. These things can be changed by pulling them out of the ground under pressure and inserting a new one, but I didn't want to risk it if there is a lot of gravel around the underground valve.
The water was backing up in the drainage area for several months and finally got high enough to freeze during winter.
It is all fixed now, but crawling around on the ground somehow injured my sciatic nerve and I haven't been the same since - two years now.