Originally Posted by meadowlm
Let me preface this post by saying I am an ....... When the valve was on and supplying water to the airstream, there would be a leak coming up out of the ground just before the concrete pad where the water connection was located (upstream of the supply valve). This leak would turn into a small stream and run as long as the valve was on. When I cut the valve off, the leak would quit flowing and the ground around the concrete pad would dry.
You are actually describing a correct operating frost free type valve. There is an engineered hole above the slug that closes on the seat at the bottom of the valve. When the valve is shut off, this drains the water in the stand pipe into a few cubic feet of prepared drainage gravel around the valve so it doesn't freeze in the stand pipe. When the valve is fully opened, the slug covers the drain hole so the hole isn't leaking. If you don't open the valve full on, the drain hole will bring forth great torrents of water down hole, causing great grief to the maintenence crew that has to repair the blown out hole and collapsed ground around the hydrant.
Your fire hydrants work this way also, they need to be fully opened. The firemen really hate mis trained folks opening fire hydrants only part way, this can cause water coming out the drain to blow out the support of the high pressure fire line. (It's fun to watch the fun on a construction job after the "underground" crew spends days looking for a leak in a hydrant and high pressure line, finally get it repaired to pass inspection on a Friday. Then that weekend the tile layers open up the hydrant part way to get a bucket or two of water and leave it open like that, washing out the compacted fill that I had just tested, the hydrant leaks again on Monday.