Maybe the tubing isn't getting hot enough. It will take more heating with that steel pipe inside to get the copper tubing hot enough for the solder to stick good. Are you using a solder gun or torch? I was able to use a 100 watt soldering gun with the brass tubing, but with the steel brake line inside, you may want to use a torch to heat the area up. But wait until the beers wear off, don't want ya burning down the house. LOL