If you have a heavy leak in your toilet, it's easy to diagnose—the faint sound of the toilet tank constantly replenishing is a dead give away. What about a slow leak? Diagnose it with food coloring.

Q Our toilet has a noise like water running, but it doesn’t flush. It just kind of seeps and my water bill has gone up. Our plumber thought a new flapper valve was needed, so we installed one. It does ...