ok.. next idea, and one that is often overlooked. take a look at the very thin vacuum hose that goes into the top of your diverter valve, it should be braided. check very carefully along it's length (make sure to follow it right to the end) for hard/brittle areas. it can be easy to miss since its a braided, but as it gets older, the heat from driving makes it very brittle and it cracks and leaks.
