I had the same problem when I de-winterized this spring.
When I winterized, I removed the screen and depressed the back flow to get anti-freeze into the city water hookup. Unfortunately, that pressure dislodged the O-ring on the backflow preventer and then it wouldn't seal properly, allowing water to leak out when using the on-board water and pump. (which I didn't discover until this spring).
I tried removing the screen and triggering the back flow preventer with my finger a couple of times but that didn't fix anything. I ended up taking the city water fixture out, disconnecting it from the plumbing and looking inside. Sure enough, the O-ring was cattywampus. I simply re-seated the O-ring and replaced everything and it now works fine. A bit of a pain to do all this, but not difficult and didn't take much handy-man know how.
As as a salute to this forum: I came here once I discovered the problem, but didn't know it was the O-Ring. Searched around and found the information I needed to fix it. I also learned that triggering that back-flow to fill the city water fill with anti-freeze isn't smart, as the pressure can and sometimes does dislodge the O-ring. Now I will fill the city water fill from the outside using a pump.