Back around the time I replaced it, I took this apart to see specifically what went wrong with it and its twin at the bottom of the slope beside the cabin. Turns out to be pretty simple. The gate is that roundish little brass thing there…
…and when everything’s working it rides up and down on the threads on the shaft that the handle turns when you turn the handle. The treaded shaft is not present in the picture above, because…
…it got stuck to the gate and broke right in two when I tried to force the valve closed. And then once the shaft is broken, one of two things can happen: corrosion and friction can hold the gate open, or gravity can close it. In the case of the valve yet to be replaced, that second thing happened which is why there’s no water to the cabin. Oh, and now that Landlady is here and on the wrong end of the muddy road, winter has returned. 
So I won’t be digging all that plumbing up just now.
Next one, take apart and use some silicone grease on it (threads and slide guide). Won’t last forever, but will last a fair number of years.
Even better, us a ball valve and turn it yearly once on and off.
Hint: Put a union on each end (whichever valve you use) for easy removal and replacement later.
I really, REALLY dislike gate valves, for just this reason. Especially in sub-surface environments. The gate valve in the water supply line to my house is currently frozen open, so I can’t shut off water to the house. The problem in replacing it is that it’s 3 feet below grade because of the freeze hazard here, so it’s going to be a major PITA replacing it. But it will be replaced with a high quality ball valve. I use ball valves for just about everything, and always have several on hand in the shop.
Here is something directly from the website of an engineering company:
Which is better: a ball valve or globe valve?
Ball valves are durable, performing well after many cycles, and reliable, closing securely even after long periods of disuse. These qualities make them an excellent choice for shutoff applications, where they are often preferred to gates and globe valves. That said, they do lack fine control in throttling applications.
Although ball valves tend to cost slightly more than gate valves of comparable quality, the minimal saving is not worth the potential issues that are likely to follow. Moreover, ball valves seal much tighter – and are therefore much less prone to leaks – than gate valves because of their 100 per cent shut off characteristics. Ball valves offer greater longevity, a lower rate of failure, and are easier to use than gate valves.
This post clears up a mystery that’s been niggling at the back of my brain for several weeks now. Thanks!
B, that’s exactly what I did with the second valve in the manifold that I rebuilt. It wasn’t broken and in fact isn’t very important but I took it apart, soaked its metal parts in vinegar in case calcium was an issue which it wasn’t, then lubed it with “sanitary” grease made for commercial chicken operations.