If a plumbing pipe continues dripping even after you have completely shut off the main water supply valves, it is usually caused by residual water draining out of the upper pipes and P-traps, or an incomplete shut-off valve that is failing to seal tightly. When water supply valves wear out internally, they often leave a slow bypass. Additionally, if the leak is on a drainage pipe rather than a supply line, turning off the water supply has no effect because wastewater remains in the upper lines. Verification: Allow residual water to drain completely into a bucket and test the shut-off valve.
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