We’ve got 4 of these on our house:
(Apparently, they were Mansfield 500s, but the product line was acquired by Prier in 2007 and is now similar to the Prier 400 series).
When the water was turned on inside the house, and the water valve was open at the hydrant, yet an attachment was not drawing water (for example, a connected hose with an end that was not open), a consistent leak occurred around the handle (some refer to it as leaking behind the handle). This meant that we couldn’t connect a water sprinkle on a timer without wasting lots of water (as the hydrant was always on).
We had two professional plumbers tell us that they’d need to tear open the finished walls on the inside of the house and replace the whole unit to fix the problem. Not believing these claims, I went in search of a solution.
One solution was to tighten the handle and was easy to perform. While that helped a bit, it wasn’t the solution we needed.
The best solution was to buy and use this kit:
Prier Products 630-7755 Service Parts Kit
It doesn’t come with instructions on the package, instead you’re direct to watch a video on YouTube with the details.
I watched the video once through and then fixed 2 of our faucets in about 15 minutes. It would have taken about 10 minutes if I had brought the correct tools with me to the faucets.
I’m no plumber, but it was easy to do the steps.
Neither faucets drip anymore. (I haven’t fixed the others yet). Hope this helps someone who’s a doubter about this working. The kit is around $10, which is far less than your plumber would charge for the trip and work.