I had the same problem when I helped someone bring their 84 GL1200 back to life. The return port on the rear MC was plugged. We had to take the MC out to get to it, and it was not easy to do. We missed the port the first 2 times, finally read up a little more and discovered the problem on the 3rd try. It is tiny, but if it's plugged the rear and one of the front will lock up.