Not to say this is your silver bullet, but 3 times I had the same problem. The metal vacuum tubes at the wheels corrode. Pull off the rubber vacuum tube from the metal one, clean it, and very lightly put a touch of silicone gasket maker thinly around the tube keeping at least 1/4" from the end of the metal tube and re-insert the rubber tube back on. Then try it.
Tuesday, January 9th, 2007 AT 6:28 PM