You’ll need to know the layout of your pipe and contour of the yard. Water without pressure will always drain to the lowest point. Each sprinkler head could be a low point. To eliminate the drainage you would need to replace each sprinkler head with ones that have a check valve. I wouldn’t worry about it, unless the drainage is causing a problem or the noise irritates you.
If you want to keep water in the pipe you'll need a check valve at every sprinkler head that drains.
This post has been edited 1 times, last edit by "worachj" (Jun 6th 2011, 6:59pm)