We had another condensation leak at our upstairs unit last night. It happened for the first time almost exactly a year ago. House was built in 1990 and the unit was replaced in 2017. I've been dumping vinegar down the drain religiously since we had the first leak last year.

What else should I be doing to keep this from happening again ? Fortunately this time we caught it early and didn't have any residual damage. The AC tech that's here said adding a J trap might help but I don't understand the logic behind it.

Try some bleach instead

What do you mean by leak? Is the primary drain line leaking? Or is it plugged and is backing up into the pan?

Plugged up and backing up into the pan.

Is it clogging on the horizontal pipe run or downstream of the cleanout/the vertical pipe section? That horizontal piece entering a 90 T doesn't look to have a lot of slope to it...

Yea that's a good point on the slope. It's backing up downstream of the vertical pipe though. It took the service tech quite a while to dislodge whatever the blockage was yesterday. He eventually had to blow 400 psi refrigerant through the pipe to get it cleared.

Bleach once a month should do it. Be careful using excessive pressure with a compressor. I've also used a shop vacuum at the discharge with a compressor at lower pressure.

We installed a cutoff switch in case the line backs up again.

Yea I definitely need to do that.