Arranging sink vent stack line
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