Drain with Air lock?

I have a bathroom drain that drains slow in my upstairs bathroom. It just started for no apparent reason. My first thought was there was a clog. There was debri in the trap and I cleaned that out, I also had debri in the outgoing pipe from the trap to the wall, so I cleaned that. However, it still won't drain which is why I thought it might be an air lock. When I put the plumbing back together the water runs fine as it is suppose to, but if I close the drain to the sink and allow it to fill and release the drain I get a couple of bubbles and then the sink doesn't drain. I can repeat the process of removing the trap and reinstalling it and get the same result. There is an approximately 1 inch diameter pipe that appears to be coming from my AC unit that "T"s into the sink drain tail before the trap. When I blow on the vent tube up in the attic at the AC I hear water in the A/C, but there is no way to blow air through the pipe in the direction of the sink without cutting the pipe. But before I do that, I was hoping someone can give me guidance on my situation. No repairs have been done to plumbing in the last two years. Thanks in advance