Upstairs Sinks Won't Drain When Used Together

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tinbling

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Mar 18, 2017
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, MN
Hello All,

I recently bought a 1979 foreclosed home and have been slowly fixing/adding items that come with the territory before making the "official" move in.

Here's my newest hurdle:
The upstairs has two bathrooms on a shared wall- one is a full bath, the other is a 3/4 for the master. The two sinks are back to back on the shared wall and drain to wall into what looks like a T-fitting (I cannot confirm if it's a cross fitting) before draining downwards.

The master sink was leaking a bit and drained very slowly with debris/sand coming up when left running for a bit. I decided to replace the p-traps on both sinks as they were 100% glued instead of being nicely threaded/compression fitted together for ease of future cleaning/vanity changing. After completing the job, I tested the master sink a few hours later and it drained really well, with no presence of debris coming up. For funsies I ran both sinks in parallel, which soon became two full bowls of water with some debris and no drainage. :eek:

My limited plumbing knowledge makes me think there isn't a vent for these drains and that one sink is using the drain of the other for air replacement. However, I can't imagine that for the last 30 odd years that the previous owners didn't ever use those two sinks at the same time... Wondering if that is the case or if there is potentially a bad blockage somewhere down that main drain?

Any and all thoughts/ideas are much appreciated!
 

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