When using the washing machine, on drain cycle, the water backs up into the bath tub, and then drains out. I have no other problems with any of the other drains or toilet. I wonder if it's the main line vent and how can I check it and unclog it? Thanks very much for any replay.

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I wonder if it's the main line vent and how can I check it and unclog it?

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It's not the main vent.
What you have is a partial blockage located downstream from the tub.

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I have no other problems with any of the other drains or toilet

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You will have in time. The reason the other fixtures don't back up is because they don't discharg with the volume and force that a washer pump does.
The solution would be to snake from the lavatory roof vent.
Put out enough snake to reach the base and 20 feet more. Good luck, Tom