Have changed kitchen sing faucet. original hot and cold fittings were 3/8 in O. D. and coupled. New hot & cold fittings were larger 1/2 outside. Got couplings that are correct fit, all brass, used pipe tape but cannot stop leak. Can't tighten any more, started torching the line. Plumbers putty? More pipe tape? Bill
HiIf your leak is from a compression fitting then back the nut off and wrap several turns of teflon tape around the sleeve. Be sure to wrap the side of the sleeve that meets up with the fitting. Then tighten it back up.
Licensed Master plumber & General contractor with over 40 years experience