Hello;

I replaced in another house a damaged American Standard 50's era toilet with another one in good condition. Prior to this replacement the customer had complaint of slow draining toilet and sink which got progressively worse over time. If it matters, the nearby sink wet vents/drains in the leg after the toilet flange about 2 feet from the toilet flange. There was another plumber who said he check out the drains, but to me it sounds like it is air locked from a clogged vent.
Any ideas are appreciated.
Thanks

Normally you would run the sink drain and see if the water level in the toilet bowl rises indicating a blockage
It is more likely a partial drain clog I would start there and then move onto venting
try removing the sink trap and see if the toilet drains better
If it does it could be venting



Edited 1 times.

It would be a rare house that did not have at least two vents out the roof. That second vent would overcome any effects from a blocked toilet vent.