Why did the febco backflow preventer fail

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.

Well, I think you have some remodeling of your sprinkler system to do. Here are some things that you need to think about.

1. There needs to be an anti-siphon type valve on each controlled sprinkler circuit.
2. These anti-siphon valves need to be mounted at an elevation that is 6 to 12 inches above the highest level of the sprinkler heads on each circuit. Not in a box buried under the ground.
3. Since all of the controlled valves will feed off of the same pressurized main water line it is convenient to install all of the valves at the same elevation.
4. If you want to hide the valves then build a wood cover that is above grade to hide then and blend in with the side of the building structure.

So if you understand the above most of your questions become moot. Pull that box of the ground and toss it away. Then re-pipe the main water feed line to above grade and create a manifold arrangement that can feed to the number of anti-siphon valves that you need (number of controlled sprinkler circuits). The manifold is best placed closer toward the building foundation. Then re-pipe the various sprinkler circuit pipes so that come up right in front of the manifold connections. The valves will affix above. When it is completed it can look something like this:

or like this:

The advantages are many. It is much easier to repair and replace things that you can get to. The anti-siphon type valve, properly deployed, is the tried and true method to provide the isolation between the pressurized potable water service and sprinker circuits. Placing your control valves high like this is also an advantage if you live in a freeze climate zone where it becomes necessary to use pressurized air purging of the lines to clear out all the water before winter. The air can be applied to each circuit at the control valve. Lastly your low voltage control wiring is above grade where is stays much dryer and less chance of wiring corrosion.