Am working on a friend's system, about 30 years old. Got all (4) zones to work, and all heads to operate properly, but have 1 zone (only) where the pressure fluctuates, to the point where some of the heads retract again, then rise as the pressure rises. Happens intermittantly, and no puddles. Took (manual) valve apart, no clogs. Took all heads off in this zone to let run out any foreign matter, no change. Any suggestions appreciated. Thanks in advence
