Water – lawn sprinkler system pressure reducer settings

To answer my own question after solving the issue...

If the problem is only at the last two heads and the first ones are fine, then it is definitely a problem occurring after the first heads and unlikely to be valve-related (though not impossible). A broken line is most likely in this case, since your last two heads are dry in the risers and flex lines (it's not the heads merely being clogged). Broken pipes can usually be revealed by looking for muddy soil after running the zone for several minutes, especially if it's a break so bad it is completely stopping your last two heads from receiving any water pressure at all. If the soil has become impacted inside the broken pipe it may be difficult for the water to escape and reveal the break.

One common method to this problem is to increase the water pressure to the break and force the water out. It will not be so much pressure that damage can be done to the PVC joints and heads (as it will only be for a few minutes nor will the PSI be any higher than the output of your main water line or jet pump). This increase in pressure can be done by capping off all the sprinkler heads in the problematic zone. This will put more force on the possible break and on the soil that may be clogged in the pipe. After several minutes the leak may reveal itself. I'm sure there are several ways to cap off sprinkler heads, but the most common and simplest method is to buy PVC threaded caps (3/4" or 1/2" depending on your risers) that screw right on to the risers.

Water pressure drops come from two sources:

1) A constriction in the line upstream, limiting the maximum inflow you can get.

2) More demand on the line, producing more outflow than the inflow can fully pressurize.

If a sprinkler zone is larger (serves too many heads), or if more zones are active at once, it will demand more water. Dividing the zone could be one solution.

If a sprinkler zone's pipe is kinked or otherwise obstructed, it may not be able to deliver as much water as is needed to fully pressurize the line.

If running the sprinklers reduces pressure at the sink, the point limiting that is going to be upstream of both, and you're probably also seeing pressure drops at the sink when you flush a toilet or run a bath. You can check for restrictions working back toward the source (valves not fully open, for example), but if the shared flow limit is what's coming into the foundation...