Here is what can happen.... if you don't use the rings.... 

We recently presented the Lucas-Milhaupt brazing rings at a Community College brazing course and they were a HUGE hit! The students found them a lot easier to use. Faster, easier & nice looking joints. They are so much faster you don’t have to worry about heating up the copper too long. Check out the photo above - this is what happens when you heat copper too long. There is a hole in the elbow about the size of a dime. It’s pretty safe to guess this joint wont be holding properly!