What keeps these from sealing is that people don't clean the hole where the o-ring seats.
I use 400 grit sand paper...put it back together (new elbows) with a coating of plumbers silicone sealant and hadn't leaked for over 50K.
The aluminum corrodes right where the o-ring is supposed to seat. It's a white powdery mess. Clean it up and it will seal.
As far as the RTV goes: put a light coating on the inside of the hole and a coating on and around the o-ring and put it together. Many people have had good luck with the RTV.
