I didn’t build it. I can see that the entry fitting & 2nd fitting is gray PVC, but that’s all I can see without a snake cam for wet environments. The behavior with the snake clearing the line temporarily, then the line being “clogged” after pulling the snake out somewhat suggests that maybe I have a shitty goffred (accordian) pipe. Maybe it’s getting kinked or folded somewhere.
I am wholly willing to bet that the sulfuric acid didn’t fix your problem but rather put a hole in the pipe itself above the blockage. I guarantee that you are now draining the contents of that drain directly into either a wall or some unseen cavity and are not actually draining out of your home/apartment/wherever.
That was my worry indeed. But I figured my next step is to replace the pipe anyway, so it was time to test the nuclear¹ option. The main sewer line in the basement is accessible just before going out to the street. I could clearly hear the water running through the main exit pipe when the kitchen sink is draining. But I can’t quite judge if the sound is gushing to the extent that it should be. So jury is out on this.
There is a cleanout at this central point where all big pipes merge. I could get more certainty if there was a way to divert all water from the main cleanout to a bucket. Then I could put 5 liters down the drain and see if 5 liters comes out. But I’m not sure now to rig up that diversion (or if it’s worthwhile).
(edit) footnote 1. I originally thought attaching the leaf blower to the drain pipes was the nuclear option… that it might blow pipes off their joints. It’s kinda like #BruceWillis in #pulpFiction in the pawn shop, where he picks up a weapon then realizes the nuke is yet to come… progressively upgrades weaponry as more options come to light.