What would cause low pressure in 4 zones - 1, 2, 4 and 6 (entire front yard and zone 6 in the back of the house)? Water barely trickles out of each sprinkler head in these zones, but pressure is fine in zones 3 and 5. There are 3 anti-siphon values (see attached drawing indicated by v v in the right side of the front yard). When I open each one counterclockwise all the way, water starts to drip out. The water stops when I close the valve.