I piped 4 zones. Three of which extend to the rear of my property. On 2 of three zones, the last sprinkler heads spray, but water shoots from the seal between the pop up part of the sprinkler head, and its casing. I replaced each head with a new one, and I am still having the same problem. Raindbird 42sa heads, rainbird valves, 1 inch poly pipe, 45 psi @ 13 gpm. I would say that I am loosing pressure but I have 1 zone with the same amount of pipe, the same length and even more heads than the ones I am having issues with. Any assistance would be greatly appreciative.