How does my automatic lawn sprinkler valve work?

Any manual shutoff should shut the zone down completely, a flow control will limit the volume of water that leaves the valve by resticting the valve diaphram movement. If neither is working they may not be operational. A zone that continually runs either has a plunger stuck in the solenoid, a ruptured diaphram in the valve, or debris between the diaphram and valve body. If all these are contained in the valve, the most likely cause would be a bad diaphram or debris keeping the valve from closing completely

the guy above me got to it before i did. sounds like either a diaprham has just gone out, or the solenoid plunger is stuck, or there is a rock or something inside the valve, not closing it all the way. like trying to close a door with your foot in it. it will not close all the way. A word of advice would be to go ahead and replace the anti-siphon valves with regular inline valves. anti-siphon valves are pretty much junk any type. As far as diagnosing the problem now, 1. check your timer's programming and make sure that its not sending out power to this zone all of the time. ( this would make the timer bad). 2. carefully open the valve up ( with the water turned off of course), and inspect the diaprham. if it has any tears or ripps in it, get a new diaphram, if not, then flush the lines out with the valve open to see if any debris is in the line( rocks pipe pieces, etc.) and clean it out. 3. take out the solenoid and make sure the plunger in it is not sticking. ( if so replace the solenoid). 4. if all else fails just simply cut out and replace the entire valve and plumb in a new one properly. personally i would use either Hunter PGV valves, Rainbird DVF 100 valves, or Irritrol 205 series valves. all are inline models. antisiphon valves tend to fail. i work on sprinkler systems full time for 95 $ per hour of labor for our company. hope this can help you out John A