I had the water stop on me this morning 1/2 way through a shower. As soon as I dried off I went down to the cistern and saw there was water dripping from the plumbing between the cistern and the pressure tank.

My set up is as follows: I have a 1550 gal. cistern with one of Bobs 1/2 hp sub pumps in it. This pump pumps to one of Bobs pressure tanks then on to the house. I have a 50# CSV-1 in line between the cistern and the pressure tank.
I don't have enough specifics yet because I was getting ready for work, it was dark, and 27 degrees outside, but what I do know is that the pressure guage on the brass tank tee was zero and there was water dripping somewhere in the 3-4 feet of plumbing just before the pressure tank, (the pvc and CSV is covered with insulation). I don't remember if the pump was running or not when I was down there, I went ahead and turned off the breaker to the sub pump anyway.
My Dad was going to come over and pull the insulation off the pipe and then turn the pump back on to see if he can figure out where the dripping was comming from.
I know that the CSV can leak water under certain situations, but what are the situations where the CSV might leak?