First, there is nothing you can do that will increase or decrease the pressure. A large pipe will carry more water than a small pipe, but the pressure is determined by the pressure in the main water supply. There is no such thing as a pressure relief in an irrigation system. If there is an open pipe, that is your problem. There are automatic drains that are supposed to open when the water is turned off and close when there is pressure in the line, but I doubt that you would have several of these and even if you did, the probability of all of them failing at the same time is nil. If would not just cap that mystery pipe, it would remain full of water when the system is winterized and break when it freezes. If you can, find where it comes from and cap it there. I'd try the 1/2" pipe before putting in 3/4". It really depends on how many gpm will be needed. The price difference between 1/2" and 3/4" PVC is so small, it was pointless to use the 1/2" to start with, but it may be enough.