I have a second floor bathroom sink drain that initially drains ok but then starts to back up into the sink after the water runs for 30 seconds or so. When the water is shut off, it slowly drains down. When the water becomes flush with the opening of the drain, the drain will belch an air bubble and then I can hear the drain clearing quickly. I have taken the following steps in an attempt to clear it but with no success.
- Removed and cleaned out the trap
- Verified that there is no obstruction above the trap
- Ran a snake down the drain line 3 or 4 times, probably 20 ft, slowly rotating it while advancing it and then again while retrieving it.