Hello, I have had this van since new in 96 and over the last two years have been warping the rear drums. Now, the master has been replaced as well as the calipers,cylinders,pads,shoes,roters and drums. I tend to think this is an issue with the proportioning valve or abs. My local chevy dealer claims there are no service buletins on this problem. Am I the only cargo van in america with this problem? 124k miles, 4.3, well maintained miles. HELP ME! I hate the shake