A friend just bought a house that is on a septic system. In the basement there is a under bar sink that had the drain line disconnected, and she wanted it reattached for use. That task was completed without any problem. However, when the upstairs bath room shower or kitchen sink is used, drain water fills up the under bar sink. The house has two external vent pipes. One in the front of the house directly over the kitchen sink and bar sink, and another in the rear of the house above the bath room. What could be the problem?

Thanks,
Don

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In the basement there is a under bar sink that had the drain line disconnected,

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Didn't anyone ever wonder why the bar sink was disconnected in the first place?
It sounds like she has a partial clog doiwn stream from the bar sink. I would snake from the bar sinks vent. ( The sink is vented isn't it? ) Somewhere in that branch there's a partial clog that's causing the back up. Get into it anyway you can and snake the drain line. Good luck, Tom