SPRINKLER TALK FORUM - You Got Questions, We've Got Answers

I have 7 zones. Each zone has 3 heads on it. Zone #6 has low water pressure. It was so low that the sprinkler heads would not raise, just bubble over. I replaced the valve and cylinoid zone 6. I also dug up the middle head were i was sure the water line was running to first, and it was. I found a cracked connection between a elbow and the side fill for the sprinkler head, So I replaced it. Now i have better pressure,but it is still low. The heads now raise, but do not shoot very far. I did however (when I had the middle sprinkler head removed) hold my thumb over the line were the middle head should have been, and the other 2 heads rose and started working great. It was like I had enough pressure for 2 heads but not 3. After I replaced the valve I noticed that there is a really small leak at one of the connections to the valve. Very small, very minor. Will this leak affect the amount of pressure to my three heads and cause the low pressure of water? My water lines are polyurethane and very old could there be a possibility that a tree root is pinching my line which would cause the low water pressure?

Any advice/suggestions are greatly appreciated!
Thanks,
Matt