I thought water flowed downhill

[tomottoe]

Hi.
I'm replacing the upstairs bathtub. Because we have to shift some plumbing I have the kitchen ceiling opened up underneath.

I've attached a rough diagram. Pipes are all 1.5in PVC.

First off, the tub (second floor) and sink (first floor) have always drained fine, and there has never been any backflow into either.

I disconnected the tub drain at point A. A few minutes later my wife ran the kitchen sink and water suddenly came pouring out at Point A. I threw a bucket underneath and we collected about a quart or 2 of water.

Then, Keystone Cops-like, my wife unwittingly dumped the bucket into the kitchen sink, and presto, more water came pouring out at Point A.

This seems to defy many natural laws to me and I'm wondering if anyone has any insight into what the problem might be, and how I might fix it.

thanks
--tom

 

[tomottoe]

I think I found the problem--the run below the tub was installed so that the end nearest the tub drain was actually lower than the end in the wall--so there was a good deal of standing water within the horizontal pipe. I think that's what got pushed out.