AC condensation line is clogged, and I can't find its outlet.

[Stokestack]

Hi all. Since I bought my house, the AC condensation has exited the house from a spout above the concrete apron in my back yard. Over time it has gotten more and more yellow, and then spectacularly rusty. I always thought this was a pretty janky setup, but with the rust I realized that something is losing a lot of material and will eventually perforate and cause a ton of damage.

Sure enough, this is the secondary drain, and the pan under the AC unit in the attic is very rusty. I shut off the system and vacuumed it out. Unfortunately I can't find the end of the primary line. I know what wall it goes down into, but it does not exit the wall outside the house or inside behind my laundry machines (which is where it goes down toward).

The primary lines exit the unit, are joined immediately, go down through a U bend, and then across the attic and down toward the laundry.

A couple of pics:

So... what would you do next? I'm leaning toward cutting the PVC to the right of the junction where I injected the water here. Any suggestions welcome!

 

[Stokestack]

See that open pipe facing upward in the background, on the main condensation line? This is almost surely the problem. I poured water down this pipe and confirmed that it flows all the way out of the attic (it could be heard traveling through a pipe in a downstairs wall).

So what's the problem? Negative air pressure created inside the system by airflow. All I had to do was stick a cap on this pipe. This video is very informative and I just happened to watch a bit more of it before I started futilely cutting this pipe apart: