Is there a specific way to, or not to, arrange or plumb the sink drain lines? Using basic fittings and short nipples I have the one line going through the wall from the trap then a left turn to a horizontal, another left vertical (down), a right horizontal for a couple feet, another left into a tee that has the vent going up approximately 5 or 6 feet, between the studs, and the other line to a second tee with another sink drain. All of this is a 1-1/2" PVC and the vent top has an air (vacuum valve) mounted at the top. Both of these sinks drain very slow! What can or should I do to remedy? My current vent for the old house I have has a 1-1/4" connection teed into the 4" cast iron that goes through the wall, below all of this. Nothing has a roof vent access, but most everything has always worked with an occasional gurgle.

Thanks,
Dave K

I would say that the aav is always going to cause it to drain slow and my concern would be if the pipe has the right amount of fall. Not too little, not too much. Research hydraulic gradient.