Watch this video from This Old House to learn how to fix a leaking system system.
1 Shut off the water to the sprinkler system.
2 Remove lid from valve box and clean out mud from inside of box with hand trowel.
3 Pack valve box with dry rags to absorb water.
4 Twist out the low-voltage solenoid valve, then wait for excess water to drain out.
5 Unscrew the bonnet from the top of the valve using a Phillips screwdriver.
6 Remove the spring and diaphragm gasket from under the bonnet.
7 Install a new valve diaphragm onto the valve.
8 Set spring on diaphragm, place bonnet on top.
9 Put steel ring on top of bonnet, then screw down the bonnet, alternately tightening the screws.
10 Replace the solenoid, and make the wire connections with waterproof wire connectors.
11 Pull rags from valve box and fill box with clean stone.
12 Turn on the water to the sprinkler system and check for leaks.