Getting wierder! The valve has a "bleed valve" on top, a long thin metal shaft that goes down the center of the valve with a plastic knob on top. By turning this 1/2 turn you can operate the valve manually. The top of the valve is a faucet type handle so you can turn the valve on manually and adjust the flow just as one would on a faucet. I have the same problem if I use the valve fully open manually and using the system with the automatic timer.
Now for the wierd= I installed a hose bib about 3/4 way between the valve and the first head when I originally installed the system. This is where I have been taking pressure readings. I have connected city water, 75 PSI, to this faucet and turned on full. The connection is thru a 1/2" line. Not enough pressure to raise the heads. I then removed the head at the END of one of the two lines in this zone and connected city water. Again, not enough pressure to raise the heads. So I have fed 75 PSI water from both ends of this line with no pressure getting to the heads! I know I do not have a leak as after two days the lines were still full of water. The heads are about six feet higher than the pump, so no drain back. Ideas? I think I will remove all the heads and flush the system with city water and 120 PSI air from my compressor. Thanks for any suggestions..