Pressure vacuum breakers, or backflow prevention devices, keep the water that has already entered sprinkler lines from flowing backward into the main supply line. This is a crucial safety feature that is required by code in many areas. Water that enters an irrigation system is considered to be non-potable, or contaminated, and is no longer fit for drinking. When the vacuum breaker begins to leak, it's probably because one or more of the internal components have failed. Replacing the internal parts of a vacuum breaker is a simple process that requires a repair kit. The kits are available at home improvement stores.
Turn both of the ball valves on the breaker 90 degrees clockwise to shut off the water.
Unscrew the bonnet cap, which is the black cap at the top of the breaker assembly.
Pull out the bonnet O-ring from around the edge of the rim. If the ring is damaged, replace it with a new one from the repair kit. If it is not damaged, put the old one back in place.
Pull out the vent assembly, which is the first piece inside the breaker.
Press down on the check retainer clip and rotate it 90 degrees to release the check spring. Remove the spring and the check disc assembly underneath.
Insert the new disc assembly in the bottom of the breaker. Place the spring on top of the disc assembly and press the spring retainer clip on it, rotating the clip 90 degrees counterclockwise until the clip is secure in the base.
Place the vent inside the assembly and screw on the bonnet cap by hand until it's tight.
Turn the ball valves on the side of the breaker 90 degrees counterclockwise to begin the flow of water. The system may leak from the top of the breaker while the system pressurizes. This will stop after about 30 seconds.