If water is leaking from the vacuum breaker portion, then there could be debris in the anti-siphon poppet. This would prevent the valve from sealing completely. The cap can easily be removed by twisting counterclockwise to inspect the internal poppet area.
Another reason is the valve may be placed at the same level or lower than the highest head within that zone. A PGV-ASV valve has to be installed higher the highest head of the zone that it is controlling. You can check the local codes from your city to find out the exact recommended height for the valve. These requirements vary by region, but it is usually around 6 or 12 inches.