One of my bathroom sinks drains slowly with the water running ( like when brushing your teeth ) but drains quite quickly if I plug the sink, fill it all the way, and release the drain plug.
When the water is running and the drain is open, the sink drains slower than it fills, but using a stopwatch, the sink drains from full, when the stopper is removed, much faster than filling it took.
I think the drainpipe is pretty simple. It goes out from the bottom of the sink, through the tap, into the wall, and straight down to the main drain line. It was snaked a few months ago, but aside from relieving the immediate clog, did nothing to improve the speed of the drain.
One thing to add, when I release the plug and allow a full sink to quickly drain, just before it finishes draining, the drain makes a somewhat loud "sucking" noise before the last 1/4 of the basin drains out. Not sure if that's relevant, but I don't hear it under normal operation, etc.
Is this some kind of venting issue or having to do with the pitch of the pipe from the trap to the wall ?
What might cause this ?