Can you please provide some possible solutions. I am experiencing VERY low pressure in one of my zones. The other zones are operating ok with high pressure and volume. The weak zone had 1" inch piping reducing to 1/2' inch with 4 pop up heads that can spray up to 45 'feet (Orbit Brand). This zone was never fully operating right with the right pressure. I checked for leaks but there were none and I even changed the piping to ALL 1" inch size. This still produced poor results. Please HELP! Thank you for your time and consideration.
Hmmm, not a lot of information to work with here. You re-piped the entire zone? You are obviously a do-it-yourselfer' who is not afraid of a shovel!
Do you know how many gallons per minute each of the four heads are delivering? I'm not going to get into uniformity of sprinkler distribution in this post, but lets look at water volume versus friction (velocity).
Assuming that your water source (city meter or well) is capable of delivering 16 gallons per minute, and you have four heads, each delivering four gallons per minute, then your 1" pipe water velocity is below five feet per second.
If the nozzle size of the lawn sprinkler heads are delivering more than four gpm, that could be a problem. When water flows faster than five feet per second in PVC pipe, the increased turbulence starts to eat up the pressure due to friction. Distance from the water source also comes into play. The farther away the heads are from the water source, the larger the pipe needs to be. Are the zones that work well using less gpm than the weak zone?
Here are some other considerations:
Is the valve opening correctly and all the way? Have you opened it manually? Is there a difference? It could be the valve. If your are willing to re-pipe an entire zone, changing out the valve will be a walk in the park for you!
Is it possible that the mainline line feeding the low pressure sprinkler valve has a kink in it like a water hose? Could a tree root have squeezed the main line? How close to the weak zone valve is a valve that works well? Maybe if the valve isn't bad you could run a new section of mainline.
You could add a valve and split the bad sprinkler zone into two zones, if that would entail less digging (sweating) than examining the mainline to the valve. In my sprinkler video I show how to do this using only one wire.
I wish I had more information to use in this troubleshooting lawn sprinklers problems post, but this should get you started.