There can be lots of places for leaks to develop under bathroom sinks.
One of the most common places for leaks to develop is where the pop-up stopper arm enters the drain. If the nut that holds this arm in place is too tight, it will be very difficult to move the control arm up and down. If the nut is too loose the control arm won’t stay up. If it is too loose a small amount of leaking can be expected.
When the attachment nut becomes completely detached, as in the following picture, a great deal of leaking will occur.
This is perhaps the easiest sink leak to fix—and once the nut is properly tightened, the lift arm will stay up as well, allowing the pop-up stopper to function again.
Charles Buell, Seattle Home Inspector
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