Heres the deal... I'm making a roll bar for my Warhead EVO. All I had lying around was some 3/16" brake line and some 1/4" copper tubing. So, the brake line wouldn't take the solder so, I decided to use the copper tubing. Well, as we all know, copper is soft. I decided to cut the copper tubing and then use the brake line to put inside the copper. Very durable and things are going well... except, I'm having a difficult time making some of the joints strong enough to hold. Some joints hold just fine and others break, no matter how many times I re solder it. Does anyone know what I'm doing wrong? Any advice of helpful hints would be great.

Thanks, Mike