a vessel with no overflow not draining fast enough. in this very specific case, i think it is good to point out that "venting" could mean before or after the P trap. In plumbing, venting means after the P trap. If you had two vessel sinks (each with no overflow) connected by a "continuous waste" onto a single P trap, they would both flow well, as air in the pipe below one sink before the P trap would get displaced into the pipe of the other sink. Newbies, search on "continuous waste" to see images. Continuous Waste Center Outlet is easiest to visualize. Continuous Waste End Outlet is the other option. If someone else more qualified wants to add to this description, please do. To add more precise plumbing terms to describe the two kinds of venting. Before the P trap and after. Water and air have to be exchanged (displace each other) for a pipe to drain well. When it drains slow, one can increase the pipe size (which allows air in the pipe to move around the water coming down) by one size, or give the air another place to go to. AAV Studor type valves are for letting air in, not for letting air out. They are used after the P trap not before. Following the logic of air displacement described above, one way to solve the slow drain is to Wye in a standpipe which would then let air move out of the way (get displaced) when water comes down the drain. The standpipe could be hidden from view; it doesn't have to be in the bathroom if you have a more appropriate place for it behind one of the walls. HTH !

The Plumbing Codes will permit up to 3 sinks of a common type and size in the same room to be connected to one common P-trap permitting that the maximum horizontal distance from the center of the sink drain opening to the P-trap is not more than 30". When a disposal is attached to one sink the sink drains are required to be connected by means of a baffled Tee. Under ordinary circumstances if the "waste arm" from the wall is midway between the sinks we would use a "Center Waste Kit" which has a horizontal baffled tee and a short horizontal line to each sink. With the common center waste kit both of the horizontal lines have a 90-degree bend on the end of the line turning upwards to attach to the sink tailpiece. When we attempt to connect a disposal with a common "Center Waste Kit" or "End Waste Kit" we run into a problem because the discharge port from the disposal is approximately 3" below the sink drain and the disposal drain port is a horizontal port. If we then attach the manufacturers supplied ¼ bend to the disposal port that will place the waste kit to low to properly connect to the waste arm from the wall. When connecting a double sink with a disposal you should first select a "Disposal Type Center Waste Kit". The disposal type kit has one horizontal line with the preformed 90-degree bend to attach to the sink opposite the disposal. The cross tube to the disposal is a straight tube with a flange on the end. You begin by discarding the formed ¼ bend that comes with your disposal unit and attach the flanged cross tube to the disposal using the rubber gasket and steel lock-ring supplied with your disposal. This will give you a cross tube coming straight off the disposal at the same level as the disposal discharge port. You then connect the baffled tee with the side opening pointing downward, which will then input into the P-trap. From the opposite side of the Tee you attach the horizontal cross tube with the preformed ¼ bend on the end, which will then connect to the sink tailpiece. To answer your second question, will it matter which way the TeeÂs are oriented? The answer is yes. An "End Waste Kit" has a vertical Tee; which only has one baffle to direct the flow from the side input to the vertical drop position. A Center Waste Kit has a Horizontal Baffled Tee which two baffles, one in each direction to direct the flow from the ends of the Tee to the side outlet. If the TeeÂs are used in the wrong configuration the baffles will cause an obstruction to flow.

You probably have a birds nest(s)/some blockage in one or more vents. Time to climb!!

1/4-turn ball valve replacements...........What to replace them with and how to replace them will depend on configuration of your existing supply-lines....the size and type of lines and existing connections. There are 1/4-turn ball valves available for literally any configuration. Plumbers have seen them all so it's easy for them to figure it out and replace them. One-timer do-it-yourselfers like me generally have to learn from scratch.....which is why I'm suggesting the plumber.....unless you're really, really interested in learning about it. It's one of those things that's easy and quick if you have experience and kind of a frustrating pain if you don't. For DIY, I would suggest shutting your water off, removing the existing valve and taking it complete to the hardware store to obtain equiv. 1/4-turn ball for your configuration. I suspect every one you have will be the same so upon figuring the first one out, you'll likely be all set to handle all the others as well. I would be surprised to learn things are otherwise at your house -- unless you have an older house that had replacements done already. In that case, you may have to examine them one-at-a-time. That's why I grump about the original contractors using the stupid el-cheapo valves in the first place. For an extra couple of bucks, they could use a decent fitting that would operate flawlessly and last for decades. As it is, many, many people like you and me have the annoyance and frustration of dealing with the inevitable failures of the inferior valves. In any event, if you're faced with a replacement....as you clearly are....my advice would be to replace with 1/4-turn ball-valve whether you choose to do it yourself or have someone else do it for you.