Entire irrigation zone lost pressure after filter cleaning

One sprinkler zone now has a weak, uneven spray pattern immediately after I cleaned the irrigation filter. Every head in that zone is affected, so I’m not planning to replace heads yet.

What’s the safest way to isolate the cause?

- A clogged or incorrectly reseated filter
- A partially closed isolation or zone valve
- A failing pressure regulator
- An underground leak on that zone

I can take static and running pressure readings before and after the backflow device and filter. Which readings would distinguish a restriction at the filter from a regulator or downstream leak? Also, should I check whether the valve solenoid and manual bleed screw are fully closed before digging?

There hasn’t been recent valve work or landscaping, but I’d also like to know whether unusually high water use would point more toward a buried leak than a pressure problem.