A PVB (pressure vacuum breaker or sprinkler vacuum breaker) is designed to protect against back siphonage of contaminated water from entering your drinking water. It uses a spring-loaded check valve which shuts tightly when pressure drops below 1 psi. and an air relief valve that opens to break a siphon when pressure drops below 1 psi. Most damage to a sprinkler vacuum breaker occurs during the winter freeze. The parts most likely to be affected according to Rick Decker of Ewing Irrigation of Las Vegas are the bonnet and the poppet assemblies.
Inspect your PVB for leakage from the air vent on top of the device. Leakage is the first sign your PVB is not functioning properly. The leakage will appear to be coming from the bell cover. The air vent is normally sealed tight by a poppet seal, which is held in place by a bonnet assembly. Leakage is a sign that the poppet and the bonnet are not sealing properly and need to be inspected.
Turn the water shut-off valve on the PVB clockwise to the "off" position.
Unscrew the bell assembly nut at the top of the bell with channel lock pliers. This will give you access to the bonnet assembly.
Unscrew by hand the bonnet assembly. This should be hand tight, but if it is not then you can also remove this with channel lock pliers. Examine the bonnet assembly for cracks. If you find any the whole assembly needs to be replaced. Set aside the old bonnet.
Inspect the poppet seal which sits directly underneath the bonnet and just lifts out freely. Look for debris that restricts free operation of the seal and flush with clean water by gently pouring a cup of water on the seal. A lack of movement suggests a bad spring is allowing water to leak out the air vent. If that is the case replace the whole assembly. The poppet seal can also lead to the check valve to fail so inspect the check valve for damage. If you need to replace them, the poppet and bonnet assembly is sold as a set.
Depress the retaining bracket ¼ inch with your fingers and rotate 90 degrees and remove the bracket. The spring and check assembly will pop out. This assembly holds the valve seal. Examine the sealing surface of the valve seal looking for debris. If you take your finger and rub it on the check valve seal and get a black smudge on your finger this is a sign to replace your seal according to Rick Decker from Ewing Irrigation, but he adds that it is very rare to have this part fail.
Unscrew the retaining screw which holds the guide in place h a Phillips screw driver. Once the guide is out then remove the seal from the holder. This whole assembly just sits in the valve body and lifts out.
Replace the old seal by inserting the new one into the holder. Position the guide in the center then screw the retaining screw through the guide and into the holder. Return this assembly to the valve body.
Place the spring on top of the check valve assembly and hold in place.
Position the retaining bracket back on by depressing a ¼ inch and then rotating 90 degrees. Make sure the spring is centered on the base of the bracket.
Rest the poppet seal on top of the check valve assembly and then insert the bonnet assembly so that guide pin enters the hole in the poppet seal. Thread the bonnet by hand until snug and the bonnet flange touches the top of the valve body.