you need to be specific about what kind of system you are talking about. Normally, the best way to make a system leakproof is to perform a proper vacuum test on it i.e. to maintain the system under vacuum for specific period of time. If there is any leak, you can find the leakage location by using nitrogen gas with soap solution (i specify nitrogen gas but you can also use air), immersed test-in which you maintain the system under pressure and see the bubble at the leak point. Higher the pressure better bubble visibility. There are also some ultrasonic leak tester which basically detect the sound made by the leak point, those which cant be detected by human ear. I suppose they are based on the fact that the soud produced by the high pressure system at leak point will lie in the supersonic range.
Now coming back to your question, In order to arrest the leak I suppose you have to carry out the necessary welding/brazing. As the high pressure steam system cant rely on the epoxy based sort of adhesives.
Also try to wait for the other eng-tips members opinion.
