I have a leak somewhere under my sink in the kitchen. I have a number of junctions/pipe/hoses there (6 lines), and one of them has a leak. It doesn't happen all the time, only when there is back pressure (like when the garbage disposal runs), and not all the time then. I've checked everything, and there is no obvious location of the leak. The leak isn't a gusher, but it's not a drip either. I've spent a fair amount of time under the sink looking for the leak, but no luck.
When I have a leak in a tire, I can put it in water and watch for the bubbles. Is there a powder or substance that I can apply to the pipes and connections that will show a leak in a similar way? If not, any suggestions on finding the source of the problem?