: I have a two story house with basement on sewer. Upstairs master toilet bowl drains, tank stays full. 2nd upstairs toilet is fine. 1st floor toilet is fine. The draining started about 2 or 3 weeks ago. No leaks visible and flushes fine. I do notice that when flushing you get a burst of bubbles out of the trap area. Shower and sink and tub are on the same stack and seem to drain fine. Also do not notice any siphoning when other items are draining. Any ideas? Other postings indicate a possible china crack that lets water down the drain and not the floor or 1st floor ceiling.
Try adding some water to the bowl only, wait a minute for any excess water to drain out, mark water line with a pencil, then wait a few hours, if it's cracked water level will lower.
A bubbling flush is sometimes indicative of a bowl that is not primed.
May not be cracked, just not repriming after flushed (rebuild first, then replace).