I used to use the 10/32 ft to 1/2" IPT adapters with a cap to cap off heads and find leaks. The last time I used those was on a job where an ex navy guy said there was a leak on one zone because it took 30 seconds longer to come on. There was a leak on a crappy glue joint but after that still the same complaint. I think I would find where the valve is where the line runs ut to the last head and then look in between. Air would be a good option.