I have an AC line tied into the sink drain just above the trap. Just above that is what appears to be some type of venting port. This is positioned upright and is open to the line. The line is currently leaking as this "vent" is overflowing and leaking into the cabinet. The sink drain itself is working fine with no blockage. The "vent" only fills and overflows when the AC unit is operating in the attic. The drain and trap assembly is brand new. The line from the vent to the drain is not. How do I stop this leaking?

Quote:

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Just above that is what appears to be some type of venting port.

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This "venting port". Does it look like any of these? (see images) Let me know. Tom

It looks like it means, you have a clear path to the attic andyour drain works. The vent should seal and overflow in the attic. The attic should have an overflow swich and pan that shuts the unit off when the pan in full.

Replace the Studor vent or AAV (Air Admittance Valve).

Better yet, replumb the ac drain outside the house.

The left image is the drain pipe and that is directly above the trap. The AC line ties into the side port. The Vent I referred to is on the AC line 6- inches above where it ties in. It looks kind of (key word) like the view you posted on the disposal arm but again it is on the ac line and not the disposal. Its vertical riser is also not as tall.

I don't believe it is desirable or allowed to be tied in with the sink. Running to outside would be better.
Our AC puts out about 15 Gallons per day.
We water plants with it.

Vent pic:

http://www.ipscorp.com/images/studor/studorback.jpg

Do a Google image search on "Studor vent"

Your AC condensate drain's connected to a branch tailpiece. This is a straight drain from the AC condensate pan to the sink drain. I'm confused!! Will someone please explain to me why a 3/4" PVC AC drain line even needs a vent? And at the terminal end yet. Even if it needed a vent, and I've seen hundreds without one but never with one, you don't vent the terminal end of a drain line.
What am I missing guys?? Educate me! Cheers, Tom

Interesting point Speedball1. Interesting point!

OP. How about some pics?