We put garden hose down into the roof vent as one of our bathroom sinks is very slow but the one right next to it is fine. This was after an earlier attempt of disconnecting the pipe under the sink. Put a mini type of hardware store flexible ridged item down the sink that is supposed to unplug it. The water from the garden hose filled up both bathroom sinks completely with water and black like sand and whatever. The other sinks & bathtub and toilets in the house drain fine. Yikes ! What the heck happened? The water slowly drained out but left this sludge. Should we snake the roof vent first?

There is double Sanitary T or Double Y connecting your 2 sinks with common drain... The blockage is below that T/Y and in the common drain... When you put hose in the vent, blockage caused the incoming water to back up into the sinks - the closest opening for accumulating water to get out...

Back to you. Milo

Hi Milo,
Thank you for getting back to me. This is the upstairs bathroom and the sink on the right drains just fine but the one next to it on the left is so slow. That's why we tried the garden hose. Could we try just disconnecting the pipe under the two sinks? Below the Y and starting from there? Or would that not be a good start? Thank you I appreciate your expertise!

First, look if there is a clean out plug somewhere under the sink or between them. If positive, unscrew it and insert 1/4" or 3/8" snake through the Clean Out inlet. If not, than undo the plumbing under the slow draining sink... Insert 1/4" or 3/8" snake and snake it through that open pipe. Be patient as it is a messy job. You will feel the blockage through the cable (snake). Work the snake in-and-out until it purges the blockage. Then, reassemble the drain and run Hot water for 10 minutes. See if that helps. Back to you. Milo

Sounds like a plan... thank you!