A/C Condensaye drain P-Trap venting problem

Discovered this weekend My primary condensate drain was not working after noticing water in the emergency / auxiliary drain pan. The line did not have a clean out before the P trap (see attached picture), nor did I have secondary drain line . I decided to remedy that problem since I would have to cut the PVC to clean it out. Cut the line cleaned the gunk blocking the condensate pan primary drain outlet (P-Trap was clean). I added a clean out before the P-trap and capped it and secondary uncapped drain line with no P trap.. Now I have a capped clean out before the P trap. Test fitted the pieces, Verfied the drain sloped away from the unit,and fired up the AC to test it. Did not work. A little experimenting revealed that if I cap the upstream clean out it will not drain. If I uncap it it drains beautifully.

My mind is baffled. It makes no sense. This is positive pressure coil, on a Trane XV80 gas furnace (not a condensing heater). I put a small inspection camera in the unit. When I cap the vent, the water just goes in a circle in front of the drain, like a whirlpool. I have let the water level get roughly as high as the bottom of the secondary drain to no avail.

Before



After

I suspect there may be a clog further on down the line. I have seen where the AC installer tapped the nearest plumbing vent line for the condensate drain. After a few years of AC condensate and Texas sunshine coming down the same plumbing vent, the slimy mold growth inside the vent would clog the vent completely - backing up into the condensate line. Easy enough to clean out with a little bleach down the line or perhaps something a little stronger followed with a good flushing with water.