Why would 2 different sprinkler zones cause pressure at the pump to drop to 0?
I ended up fixing this issue by capping off the heads and turning the system on for hours at a time as opposed to just a few minutes. This extended time running the system while capped off allowed buried line breaks to be revealed. There were multiple line breaks and crushed heads run over by vehicles in the past. I fixed all this last summer. As a preventative for future breaks I placed concrete doughnuts around each head.