Call a plumber and he replaced a gasket behind the overflow. He even open the access panel to the pipe and check. Still leak when taking bath. What could the plumber have missed?

I assume you mean that it only leaks when using bath tub and only when water flows out overflow. Open access panel. Fill tub and let it overflow. Watch to see if water comes out around overflow. If it does, call plumber back. He may only need to tighten the screws that hold overflow pipe to tub.

If water is coming from anywhere in that area you should be able to see it. If you see no leak, open drain. If leak begins after opening drain then leak is around drain flange or in drain piping.

I assume you mean that it only leaks when using bath tub and only when water flows out overflow. Open access panel. Fill tub and let it overflow. Watch to see if water comes out around overflow. If it does, call plumber back. He may only need to tighten the screws that hold overflow pipe to tub.

If water is coming from anywhere in that area you should be able to see it. If you see no leak, open drain. If leak begins after opening drain then leak is around drain flange or in drain piping.

Thanks! I'll try that