le n, 3 times. Then I Googled stubborn drains, unhooked the U-bend, and found a wodge of stuff right at the bottom of the bend that the bs/v/hw had penetrated but not been able to dissolve. I cleared that out, reattached it, and the sink drained splendidly - exactly once. Re-unhooked it, put the improvised snake up the next section of pipe and hit something extremely tarry and gunky just past that section, where the pipe disappears into the wall.
If I could get that section off I could probably bs/v the next bit, then reattach everything and flush with hot water, but it's angled weirdly and there's no way I'm going to attempt it on my own; if someone's going to break the wall tiles and the pipes and incur the landlady's wrath on my birthday, I'd rather it be Maintenance Guy Ray.
I also tried reconnecting everything, flushing with boiling water until I had an inch of standing water, and then plunging the sink, and that brought up a lot of gray sludge and silt but didn't budge the far-in tarry clog. So I think I'm at the limit of what I can do without corrosive chemicals or breaking the wall, and it's time to call Ray.
OTOH, I was pretty chuffed to get the U-bend out, cleaned, and reattached. If it came with health insurance, I would be ridiculously happy with a maintenance gal/building manager job (the second most amazing place I've ever seen in SF was a freestanding cottage in the middle of a big apartment complex, occupied by the maintenance couple for the complex who lived there rent-free in return for being on 24/7 call; they were bookish geeky types who just wanted to live cheaply and fix things and read, and the cottage looked exactly like all the on-campus faculty housing from my college but better - bookshelves built into the stairway and all the stairs).