I have 14 zones which work fine except for zone 6. When it starts water starts coming out of backflow preventer. I already changed the valve parts and solenoid. Is there any thing else I can check?
It sounds like you have a blockage in that zone and the water is dispersed through the backflow.
Try taking the heads off at the beginning of the zone. Also look for damp ground where a brake may have occurred. If you had a break and repaired it there may be debris in the pipe that has been compressed by the water and is now a solid block.