Improper venting leads to sewage back up

As a result of a remodeling job, the workers capped off a sewer pipe that was once properly vented out the roof. They must not have realized that I had a basement bar sink that is a part of same set of drain pipes.
My symptom was that the bar sink would no longer drain properly. This made sense to me: there needs to be a vent so that air can be drawn in to help the system drain.
The OTHER symptom was sewage getting sucked through the bar sink P- trap and belching up a foul sludge into the sink (I've remodeled my bathrooms before and have had my head nearly in the toilet sewer hole and yet I've NEVER smelled a smell like this). This was strange, this part of the sewer circuit is not active and should not have material in it.
Does the vent also serve other purposes other that proper drainage? Without it, does it create some sort of, I don't know, vacuum that pulls sewage towards pipes that would otherwise be clear?
Connecting this pipe back up with the vent in the roof would be a considerable effort. So I put an indoor vent cap on the end of the pipe. It supposedly lets air in but won't allow are to come out. I don't get sludge backups anymore but I do get sewer smell. I take it that these indoor vent caps don't work very well.
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djh wrote:

The "vent cap" is a mechanical vent that should not allow sewer gas to escape. But it may not be an adequate solution to the overall venting problem.
Not nearly enough info. You would have to post a diagram of the complete venting/drainage layout.
In the end, a proper roof vent may need to be installed.
Jim
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Depending on the plumbing layout, there must be some water/sludge that settles in the pipe leading to the bar sink, when upstairs facilities particularly a toilet is flushed, enough pressure must be forcing this sludge up and into the sink. The exterior vent that was connected to the sink must have relieved the air pressure in the drain line and did it's job, however, it sounds like the bar sink line is not draining well, and may be allowing sewage to back up into its line causing the foul burps. The pipe may need to be properly sloped towards your main drain line as the sludge burps may end up clogging any new vent line and/or may continue to discharge into the sink. Either that or your main sewer line is partially blocked preventing the sewage from exiting in a steady flow, instead it may be causing a backup into your sink. You may want to get a plumber out to check for any blockage between your stack and the street sewer.

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Thanks for your responses! I threw together a crude diagram and posted it below.
http://picasaweb.google.com/danjhiggi/SentByOthers/photo#5093091318589685202
The theory that waste is working into the bar sink line and pressure is pushing it through the P-trap is interesting. I'm guessing that the waste is from the kitchen and that bathroom waste isn't going to work its way all the way back to the other side of the house.
I'm holding off bringing in a plumber until I better understand the problem. The last plumber I had (for a different problem) wasn't very thoughtful about root causes.
-dan
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snipped-for-privacy@gmail.com says...

Is the smell constant, or does it come and go with other drain use?
If it's constant, have you taken apart the sink trap and cleaned the rest of the drain very thoroughly? Is it possible you're smelling something that got left behind by one of the sludge burps?
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snipped-for-privacy@phred.org is Joshua Putnam
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I appreciate the comments. This has been helpful.

The plumbing connects (under the concrete) in such a way that kitchen waste can easily back up into the pipes leading to the bar sink. Subsequent pressure from the active kitchen pipes apply pressure on the wayward waste and eventually push it towards the sink.
Reconnecting the vent from the bar sink may help but it may not. The bar sink is close enough to the kitchen line to benefit from its vent.
I'm thinking about re-running the bar sink to the vertical kitchen line so that its properly sloped plus use a sloping connecting piece. This way no waste can back into it. -dan
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djh wrote:

Oh, if you can make that connection it may solve it. Use a WYE or a long-turn TEE-WYE for the connection. The mechanical vent you already have will admit air as the kitchen sink drains, preventing siphonage of the bar sink trap. You may hear the mech vent "chatter" as the kitchen drains.
Jim
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