Actually, when I use the bathroom sink, it pours, too!
On Sunday night, I got sidetracked and started cleaning the bathroom. Then I noticed the bathroom sink was draining slowly again, so I decided to clear the drain with a little Drano. And that's when we heard the melodic sound of water splashing onto the dirt in the crawlspace underneath the bathroom. A quick glance downstairs confirmed that the bathroom sink drain was pouring water. No other pipes have been affected.
On Monday, MNB and I got changed into work clothes and ventured into the crawlspace. It was pretty obvious that the pipe we needed to fix was not down there, but rather inside the bathroom wall.
Step 1: Turn off the water to the sink and move it.
Step 1: Turn off the water to the sink.
Step 1: No, really, turn off the %$*#ing water to the sink.
The old valves were stuck in the open position. Nothing would make them turn. So, Step 1: Crawl under the house to reach the water main and turn off the water to the entire house. I made MNB do that.
With a bit of prying, sawing and smashing, we were able to see the pipes. The problem seems to be that the big old iron drain pipe is, for lack of a better word, broken. Not cracked. Not a little hole. Broken clean through. Since the drain enters the pipe below the break, it may have been broken for a while. I think that the pipe can handle a small stream of water, but larger amounts can't drain quickly and thus back up to where the break is and spill out.
Do you know what the plumbing books say about iron pipes? Call a professional. Fortunately, the sellers bought a 1-year warranty on the house for me, and according to the paperwork, plumbing is covered. Even better for the plumbers, I'm not going to ask them to patch up the hole in the wall. I plan on ripping out the wall entirely when I renovate the bathroom this summer.