I tiled the upstairs bathroom floor, and put the toilet back in yesterday. It was removed for about 3 weeks. Now the water level in the bowl slowly decreases over a couple of hours.
I took out the toilet again this morning, no sign of leakage anywhere, wax seal looks good. Can't see any cracks in toilet.
I risked my life climbing onto the roof. Couldn't see any obstructions in the vent pipe with a flashlight. Flushed water down the vent with a water hose for five minutes. Didn't back up, seems to drain fine.
Reinstalled, and water level still decreases. Flushes just fine. Water does not leak from the tank at all. Downstairs toilet works perfectly. All other drains (sinks, washing machine, etc) drain just fine.
What am I missing here? Who wants the pos rep?