Based on the fact that you say the valve opens fully when you manually open the valve, it appears to me that for some reason the solinoid plunger may not be coming up all the way when it is electrically actuated to allow the water to exit the upper chamber at the proper rate. I'm not sure what you did to convert your valves as you say. Do you have the correct solinoid? and if you have other zones do they operate properly? Maybe you just have a stuck solinoid. Try another one, or switch with another valve.