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

I have been working on the sprinklers for two or three weeks now, the back yard looks great, but the front yard is terrible, and not getting better. We have very low pressure, and after fixing multiple problems in the line, it is better, but still much lower than it should be. Here is a list of what we have and the problems we've fixed; after each fix we checked the line and pressure is still low:

Replaced two cracked tees, both of which were causing leaks, and are no longer leaking.
Replaced a leaking coupler, which no longer leaks.
Found a spot in the line where the pipe wasn't even attached, and coupled it, no more problem. (This was probably here since the previous owner)
Replaced all the heads and risers with new ones.
Unclogged the pipe in a location where the head was not spraying; the line there is now clear.

We have 3/4" pipe, everything is correctly attached with PVC cement, the valve is open, but the water pressure is still lower than it should be. Some water sprays out of the heads, but most of it leaks down from the top of the head rather than spraying. The heads are brand new, the filters are clean. After running the sprinklers and turning them off, we saw that water was still leaking from the top of the heads (as is normal) however after unscrewing one head from its base we had a nice fountain of water for a good 30 seconds spewing from the connection point. What would the problem be? Is it a problem with the valve, is there a crack somewhere else in the pipe? Basically the question now is what would be the next thing to replace or test, considering what we have done so far and the results we have achieved?