I too used Teflon tape and dope working in the oil/chemical field and never had a problem getting a seal. You might have a cracked tube causing the leak. By the way, Teflon is good up to 400F so it's not a heat related leak. I'd pull the tube, clean it and inspect it for a crack. If nothing can be found, use tape this time. There seems to be Teflon tape on the market that isn't the bast quality. The good tape should have the consistency of Saran Wrap and not like cardboard. Wrap the tape on the opposite way the threads are cut and start back 1 thread with the tape. This should prevent tape from tearing loose and getting into the system.