Bad zone - obstruction?

[n2sullivan]

When I started up my system this year I found I zone with little pressure making it to the heads. First thing I did was check the valve. Its good, I went ahead and tried a different valve on that zone. Solenoid is functioning fine too. Water pressure to the valve is good. But down at the heads there is barely enough pressure to get them to rise a couple of inches and spray water maybe a foot. My best guess is an obstruction somewhere. Any suggestions on how to check this and/or remove it?

A little more info: I blew out the system before first freeze and we have been frost free for a little while. Last season pressure was fine.

[umfhockey66]

If the pressure is good all the way around and the valve is good, let it run for a while and check for a leak. Is there low pressure to all the heads or just some of them? If its some of them its pretty easy to track down, if its all of them , its probably a leak let it run for a few minutes, walk around the zone back and forth, and eventually you will probably hear a squish, keep an eye around the driveway and sidewalks if the zone runs under them, those areas are usually the low spots and any water left over after the blow out will tend to accumulate there.