Bathroom sink tailpipe leak

[sheahasu]

I decided to update the faucets in the master bath. Old faucets used metal seat and tailpipe. The new one uses plastic. Had the whole thing in just fine, but it was leaking around gasket. Finally determined that the directions did not include a step for sealing the seat. I tried putty, but it didn't want to seal. Online research led me to believe that the seat being plastic instead of metal could be the problem. I switched to silicone. Reinstalled all, let silicone cure for 24 hours. Ran water--still leaks, though not as bad. I have tried adding teflon tape to the threads. Taking teflon tape off the threads. Using the old gasket and washer on the new plastic tail pipe(the new one doesn't have a washer, it's sort of "built-in") After messing with it repeatedly, I managed to loosen the seal, so I had to start all over. I decided to try using the old metal pipe with its metal washer with the new plastic seat. If that doesn't work, I'm at my wits end. I have another identical faucet to replace, but I don't want to do it until I get this bug figured out. I can't afford 2 non-working faucets!

Does anyone have any ideas what might be causing the leak at the gasket and what I can do to make a tighter seal?

[Pulpo]

Changing the drain isn't always necessary when you change the faucet but now that you already did it, you have to take it apart again. Look at it carefully. Take your time & don't rush. Make sure that you didn't crack the sink or the plastic pipe. Then clean everything as best you can. Remove all the old putty, tape, glue & whatever else is there.

Put a lot of putty under the seat. Drop it through the sink. There should be a heavy duty rubber gasket that threads just like the retaining nut. That has to be tight & centered so it seals the hole from the bottom. In other words, the seat has to be sealed from top & bottom. Then put the retaining nut on it. Once you have that set properly, there shouldn't be any leaks from that section. Remove the excess putty after your done. Maks sure everything is dry & check it again.

[SDPlumber]

If using silicone on the flange and also the rubber gasket underneath the sink doesn't work I'd say something has got to be cracked or have a hole in it.