This component can go bad with use and to repair it you will most likely need a new anti siphon rebuild kit that is brand specific to your faucet.
Frost free anti siphon outdoor faucet leaking.
Leaking from anti syphon fitting another place a frost proof faucet can leak is from the anti siphon portion of the valve if the faucet has one.
If your frost proof is set up properly you should not need to use the old water valve any longer.
Remove the knob s retaining screw or nut and pull off the knob.
In this video mark donovan of http www homeadditionplus com shows how to fix a anti siphon valve on a faucet.
Anti siphon valves are simple devices and leaks aren t difficult to diagnose and repair.
Put the knob back on and turn it counterclockwise to remove the valve stem.
They should also be angled down slightly so water runs out of it when shut.
If your faucet leaks out of the spout when it s turned off the washer is bad.
Before removing a screw on valve turn off the faucet release water from the hose and detach the hose from the valve.
Remove the handle and nut photo 1 and pull the stem from the faucet photo 2.
An anti siphon valve normalizes the pressure inside the pipes when you turn off the water to prevent it from flowing backward.
On frost proof faucets the washer is located on the end of the long faucet stem.
2 unscrew the plastic cap covering an anti siphon valve permanently installed.
Some freeze proof faucet models have two nuts to remove.
Frost proof faucets are designed to shut off inside the envelope of the home so it is warm enough not to freeze.
When installed on an outdoor faucet the valve also allows residual water in the faucet to drain which prevents faucet and pipe damage from frozen water in winter.