We have a two basin kitchen sink. There is an inline air vent attached on the drain system before the trap.Water drains OK for about 5 secs then starts backing up and comes out through the two holes in the inline air vent?

If possible, spin the air vent out and then snake the drain line through that access. If you have a full size cleanout under the sink then, of course, snake the drain through that. Install a new air vent when done unclogging the sink.

Mark

inline air vent attached on the drain system before the trap
Hopefully you mean after the trap. That is, between the trap and the wall.
Snake the line as Mark said and then replace the vent. Water should never come out the vent, even if the line is blocked.

Hopefully you mean after the trap. That is, between the trap and the wall.
Snake the line as Mark said and then replace the vent. Water should never come out the vent, even if the line is blocked.

Ah, I wonder if you have hit on a big flaw in the design.
Yes, water does come out the inline air vent and it is situated before the trap.
Let me try to describe the inline air vent, so we're sure we are talking about the same piece.The inline air vent is about 4"high, color black, circular and fits onto a straight line of pipe . At the top of it is about a 1/2 " cavity / space which has two holes. It is out of those holes the water backs up out of
It is a two basin sink. The lines comes down from the two basins, then horizontally go towards each other, [that's where the tee is] joining into a tee. The tee then leads to the trap, which turn down to a pipe going down through the floor to meet a 2" drain pipe in the basement, which runs 26' horizontally before connecting with the 4" main drain. Is this design a problem?

It is a two basin sink. The lines comes down from the two basins, then horizontally go towards each other, [that's where the tee is] joining into a tee. The tee then leads to the trap, which turn down to a pipe going down through the floor to meet a 2" drain pipe in the basement, which runs 26' horizontally before connecting with the 4" main drain. Is this design a problem?

You didn't say where the AAV is.

The AAV (vent) should be after the trap (as the water flows). Logically it should at the top of the pipe going down through the floor.

The AAV should let air into the pipe. It should not let water out. It appears to be defective.

You also appear to have a blockage. Remove the AAV, run a snake down the line to clear the blockage. Put a new AAV in.

Some of the older black ones are spring loaded and over the years corrode the spring. The AAV should only let air in but it also sounds like you have a clog. Get a Studor vent.

Hello Tipperary7,

Remove Trap, clean it. Take 3/8" snake, insert it into the drain pipe and push it in entire length. Do it several times. Reinstall trap. Turn hot water ON and let it run for 10 minutes. Your drain service should be reinstalled

Hope that helps

Milo