Dirt and debris trapped in the valve may cause it to "weep." The telltale sign of a weeping valve is excessive puddling at the lowest sprinkler after the valves have shut off.
To check for excessive dirt, debris or algae buildup, turn the water off,remove the valve bonnet, and check the screens for contamination. Some valves have filter screens directly below the solenoid, which can be removed with a small screwdriver or coin. Flush the screens with water to remove any debris.
This is also a good time to check the diaphragm and valve seat for debris,wear or deterioration (note that some valves have these two components molded into one piece). The diaphragm, which is a large, rubber-like,flexible disc, is subject to deterioration. It can be nicked or torn by a trapped pebble or a build-up of grit.
The valve seat is the lower sealing surface in the valve body. Inspect it for nicks by running your finger over the lip of the valve seat. Replace the valve body if the valve seat is damaged. Check the diaphragm and the valve seat for cracks and wear. Replace them if they show signs of wear or deterioration. Reassemble the valve, turn on the water, and manually operate the irrigation controller to make sure everything is working properly.
Solenoids If you have checked the water supply, the power supply and the diaphragm and valve seat, and the valve is still malfunctioning, usually the only possibility left is a faulty solenoid.