Valves can leak for a variety of reasons, including:

• The valve is not fully closed (e.g., due to dirt, debris, or some other obstruction).
• The valve is damaged. Damage to either the seat or the seal can cause leakage.
• The valve is not designed to close 100%. Valves that are designed for precise control during throttling may not have excellent on/off capabilities.
• The valve is the wrong size for the project.