Great question — answering as the UphillTech team:
Disassemble the fitting and try RTV silicone from an auto parts store. Allow the joint to set up overnight if possible. Brass fittings are hard to seal, and do not tighten too much. Brass is soft and will stretch. If you have tightened to where almost no thread is showing on the connection, you may have to replace the fittings. No teflon or pipe dope, just RTV silicone.
Full answer by d.george: https://diy.stackexchange.com/a/110895 (CC BY-SA 3.0).
Need it done for your business? UphillTech.
