2 years ago, the last sprinkler head in one of my zones stopped working completely. I took the head off and turned the zone on and still no water came out. The next year, the 2nd to last head stopped working completely. And in the last few weeks the 3rd to last has stopped working completely. I've taken the heads off all 3 and still no water comes out.

There has got to be a crack somewhere, what is the best way to try to find it and replace the pipe? Where is the most likely location of the crack, near the last sprinkler head or somewhere before the 3rd to last one?