I was hoping this would be a simple project but it's turning into kind of a monster:

I replaced a couple old brass manual jar-top valves with plastic automatic valves. The old valves had essentially a built-in 3/4" union on one side, but the new valves (all I could find) have two plain threaded female connectors. The inlet side is galvanized pipe, the outlet side is class 125 PVC. I decided the easiest way to hook up the valves would be to use 3/4" unions on the outlet side. However after hooking everything up, flow to the sprinkler heads (rotors) seems to be significantly reduced, due to a constriction in the unions, and I'm not getting complete coverage any more.

Are unions available that would have less flow restriction? The ones I have seem to be closer to schedule 80 inner diameter. Or should I do this hook-up without unions? Based on my history of mistakes, I'd prefer serviceable connections. I tried using schedule 40 compression fittings and that helped but didn't seem like a safe solution -- maybe it was due to my incompetent installation but the fittings creeped under pressure. The old brass valves were a non-standard design so I couldn't retrofit the existing valves with an automatic actuator.