I have a customer that has asked me to look at his sprinkler system. It is a very old system, most of the heads are/were the old metal ones you can't even buy anymore. The problem is that the heads wont pop up in one of the zones. It is 3/4 in. pipe with 12 heads in the zone. I can't seem to find any major leaks or breaks in the zone. My guess is that over the years when the old heads have been replaced with newer ones, it has changed the output of the zone and there is not enough water to run all 12 heads. Any Ideas or solutions?