I have a similar compressor and have also seen problems like this
sometimes when blowing out my system. And I've also seen problems
where water mysteriously sprays out of the backflow preventer during
Spring start up.

I'm not an expert on what's inside the backflow preventers, but
essentially it relies on a moving plastic object being pushed by water
to close a passage. I think the problem is that air has a lot less
mass, and the widget may not move as easy with air as it would getting
hit with water. So, I think it must get stuck mid-way, with the air
continuing to go around it. This is probably less of an issue with a
compressor that can deliver more CFM. The ones we have are marginal,
though they will do the job.


I'd try the following:


1 - Go back to running water through it again. This should get the
valve operating properly again. Then you can try air again.


2 - Rap the backflow valve with a hammer to try to free it up without
air on it, then apply air and rap again if still misbehaving.


3 - Take the top cover off the backflow valve. On mine, the thing
comes apart easily with just 3 screws and you can pull out part of the
mechanism. Every time I've done that and put it back together, it's
gone back to behaving again.


Also, the total size of the system is usually not a factor for a home
system. They are set up on zones. You only need to have the
capacity to do one zone at a time. Whether there are 3 zones or 12
doesn't matter. What does matter primarily is the nozzle size and how
many heads per zone.