Done!
Thanks for the support!
Blew the 7 zones out today through the backflow preventer testcock. Worked fine. My compressor is just big enough to maintain around 55 to 60#.
During the process, I found that there were good-sized air bubbles coming up through water that was in the 4-valve box. I figure that means water has been leaking from a connection in there 24/7, perhaps for months. And the pressure to all the zones must have been lower than normal. The leak is underground just outside the box. I'll have to dig it up in the Spring. hmm..... I hope I shouldn't be concerned that water from rain/snow will accumulate in the box, leak back into the pipe, freeze and crack it (??)
Thanks again!
Frank