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Ok guys - I'm remodeling bath #2 at my parents house and am hitting the exact same problem that stumped me a year ago on bath #1. Unfortunately, I don't remember what ended up solving it then, just remember spending the better part of a day on my back upside down cursing at it.
I bought a 1 piece cast sink / vanity top assembly, and a Moen faucet / drain kit.
Everything went together fine, except the sink tailpiece leaks.
Now, the ONLY place I have a leak is at the very top underneath - where the large rubber donut meets the bottom of the sink. This is ironic because there are 8 other compression fittings that are all perfect. Since these are all plastic-plastic connections, I didn't use any sealant except the putty between the drain ring and the sink itself up top.
What did I do wrong here? I can't tell if it leaking down the tailpiece threads, or leaking around the donut washer. Did I overtighten? Undertighten? If it is leaking down the threads, what's the right way to seal them?
Your tailpiece is two pieces that connect with fine threads. Where they connect there is a plastic cone shaped washer. Throw it away and use teflon paste on the threads. Also apply teflon paste liberally on the rubber washer that seats to the sink. If there is a thread it don't hurt to dope it.
Sounds like you have it in order correctly. Cast sinks can be difficult as they may not be formed perfectly to provide a good seal. Did you leave enough thread when you adjusted the drain plug and screwed it inot the tail piece, or is it a single piece? Lots of putty? Is the ball valve nut tight?
VERY common problem. Run down to Home Depot's plumbing department and get a tube of Recorseal Tplus2 teflon pipe thread compound. Use it around the threads and the seal. Many people think no compound is needed on threaded plastic fittings, they will often have slow leaks if you don't use some.
Guys, sorry, when I said cast sink, I mean cast from some material that fell from outer space. (I don't know what it is, corian looking stuff)
It seems the leak is somewhere on/around the rubber mack washer (?) that seals the tailpiece to the sink. My suspicion is that the washer is sealing fine against the sink, but not against the threaded pipe, allowing water to seep down the pipe threads.
After doing more research online it does seem that teflon paste is probably the answer as you guys suggested above... this strikes me as odd as I've never in my life used teflon paste on anything but I'm willing to give it a shot. I do have lots of plumber's putty and teflon tape lying around - I am resisting the urge to use those instead and will probably make it to HD later today.
If it's got a thread dope it. I learned that a long time ago, theres no money to be made on call-backs. Sounds like your leak is where the chrome ring screws to the assembly for want of dope. Always "paint" the rubber washer with dope where it seals to the sink.
I think you got a putty problem, that's what seals the bottom of the sink. You might check your holes in the tailpiece to make sure they are totally inside of the 2 walls in the sink bottom for your overflow.
Do not put dope or putty on the threads where the smash washer(big rubber washer)is, put some muscle on it and tighten it down, then tighten it down some more. Also the washer should be cone shaped, make sure the cone side goes up into the sink as it tightens it will force it around the drain. Ive seen cast iron sinks seal with chunks taken out around the hole in the bottom. Tighten it down it will seal. Also the putty in the bowl of the sink is only there so when you plug the sink the water doesnt drain out. If you look at the piece that goes down through the sink you will see holes below the drain. That is for water that goes down the overflow to drain into the drain. So as you can see the putty does nothing. The real seal is the smash washer, tighten the hell out of it.
It sounds like you may have a bad cast on that manufactured marble top (I'm assuming that's what you have when you say "corian-looking stuff"... most of those precast tops are manufactured composites of some sort.
The putty job inside the bowl has nothing to do with your leak underneath - that's just to keep the water in the bowl when the stopper is in. The leak from the mack washer underneath is probably from a bad cast. They typically cast that unit as one piece, then grind off the outlet to get a flat, smooth sealing surface. You may have a burr or an imperfection of some sort... very common on those tops.
If it leaks again, take it apart and smooth out that sealing surface with some emery cloth or a fine sandpaper. It probably cut the mack washer and you may need a new one.
Put the mack washer on DRY - no sealant. Ever.
Good luck, PM me offline if you have any questions.