Hmmm, thats a strange one, so your saying that when you run water down the Kitchen Sink, water comes up through the drain into your bath tub ?
You may need to get a pro in on this one.
Or you may be able to hire what they call a "snake" from your local hardware store. basically you take off the drain cover and insert this snake into the pipe and it goes through and sucks up everything in its path to clean the pipe out.
As a side note I usualyl find the Drano Powder works better than the Gel.
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