Slow bathroom sink drain in a two family building

[TCL]

I have a slow bathroom sink drain in the 2nd floor unit of a two family building. The drain can’t keep up with the flow of the faucet. Every other drain in the building (2 tubs, 2 kitchen sinks and the other bath sink) flows well. The fixture drains and waste stacks were replaced with PVC in 2001. Cleaning the stopper and p trap showed no improvement. One odd characteristic is when the sink bowl is filled to the overflow and the stopper is raised via the rod, the drain evacuates normally without hesitation, however if the bowl is less than ¾ to ½ full or less, it is extremely slow.
Any advice or insight would be appreciated. Thanks

 

[Cacher_Chick]

You mention that the drains were replaced but nothing about if their installation is correct. I would start at the trap arm and check the pitch of each horizontal run. Anything less than 1/4" per foot can cause a problem.

 

[Sponsor]

 

[TCL]

Thank you for your reply. The drains were correctly installed in 2001; licensed plumber with rough and finish inspections by a local city plumbing inspector, 1/4" per foot drop. The issue arose within the last 30 days.

 

[Cacher_Chick]

Have you actually pulled the stopper rod out and looked through the tailpiece?
If anyone in the house has long hair, the chances of of a drain clog go up by about 80%.

 

[hj]

The overflow "pipe" is probably plugged so unless you have a full sink of water, there is not enough pressure to force the "air bubble" out of the drain to allow full flow.

 

[TCL]

Thanks again for your reply. As stated in the original post, I have pulled and cleared both the stopper and p trap.

 

[TCL]

Thank you hj, I have not cleaned the overflow yet. Are there any techniques or tricks to clear it? The sink has a small 1/4 to 1/2 in hole near the left outer rim of the bowl; would you snake it or use something else around the stopper area of the throat? Thanks again for your insight.