Replaced the toilet fill valve, flapper and supply line on an old toilet at my daughter's house. Can't seem to get the supply line to stop dripping. Driving me nuts! I bought a braided stainless steel line and screwed it in place. Leaked. Removed it, added tape to the fill valve threads, still leaked. Added putty to the inside of the stainless steel line, still leaked. What am I missing here?

Let's see if I understand your problem.

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I bought a braided stainless steel line and screwed it in place.

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I assume that end of the toilet supply's leaking from the connection under the tank to the ballcock and not the compression nut on the angle stop. Am I correct so far?
If that's the case you have crossthreaded the supply line into the ballcock. The threads do not contain any pressure so tape and pipe sealant isn't needed. The washer contains the pressure. Don't beat yourself up over this. Plastic threads are easy to get crossthreaded. Go back and either start the threads correctly ( without the tape and sealant) or if the threads are too messed up simply replace the ballcock. Good luck, Tom

If you have a leak where the braided supply line connects to the fill valve, you must have cross threaded it when making the connection. The braided stainless supply lines don't leak. They use "O" rings to make the seal. Since it will be the threads on the plastic fill valve that are damaged, replace the fill valve.

Boy, you guys are good. Same answer, 5 minutes apart.

Great answers above. Just one other thing to check. Are you sure the leak isn't from the tank itself and not the supply line? If you installed a new fill valve, are you sure you tightened the anchoring nut enough.