Bathroom sink drains slowly enough that bowl starts to fill up after about 20 seconds of water running fully.

Removed the U-trap, found nothing blocking it, tried running water with a bucket under drain/removed trap and the flow was back to normal: i.e. it seems like the drain stop mechanism is not to blame.

Ran a drain snake down at least 10 feet several times, found no resistance or goop on the snake's head.

Unencumbered by any plumbing knowledge, my current theory is that there's a plug of goop down there and the snake has bored through it - but what's enough room for the snake to pass is still insufficient room for a normal drainage flow.

Does anybody have any thoughts on what's next?

a professional rooter guy would probably be willing to do this work.
If you have old cast iron or galvanized drains it might be time to consider re-plumbing with ABS or PVC.



Edited 1 times.

Would you call 50 years "Old"?

--
PeteCresswell



Edited 1 times.

if it takes a full 20 seconds for the water to back up, i would say the clog is farther away than 10 feet.
you need a longer snake wire.

Thanks for the 20 feet observation.

I was just guessing on the snake depth.

Now I'll do it again and measure.