Here are some tips for valve repair.
Turn the irrigation controller to "System Off" so it will not cycle a program during valve maintenance.
Shut off the main water supply. Opening a valve under pressure is dangerous and could cause injury.
Unscrew the solenoid (cylinder with two wires) by turning counterclockwise.
Unthread the outer ring of the valve by turning counterclockwise. If there is too much resistance by hand, gently use a pair of large channel locks.
The diaphragm may be sitting in the body or captured in the bonnet. Remove the bonnet, diaphragm, spring, and support ring. Note the orientation of the individual parts so you can properly reassemble them. Examine the diaphragm and spring. The diaphragm should be clean and free of wrinkles, tears, and perforations. The seal should be clean and free of nicks, dents, and abrasions. Wash the diaphragm with clean, fresh water to remove any debris that may have been deposited there. The diaphragm spring should be intact and bounce back when compressed.
Examine the valve body and remove any debris, pebbles, or dirty water. The diaphragm seat in the body should be clean and free of nicks, grooves, or abrasions. Next, check the solenoid ports for any smaller debris. This can be done by pushing a small piece of wire through hole to free any debris..
Reassemble the valve in the reverse order of how it was taken apart. The diaphragm should sit with the lip in the matching groove on both the body and the bonnet. The jar-top ring should be hand-tightened only. Over-tightening can damage the valve.