.A company I worked for had this same problem with several of their early 80s pickups. The problem usually happened when turning and applying the brakes at the same time. The problem was diagnosed as being a port in the booster being blocked when using the brakes, effectivly putting the steering in a hydraulic lock. This was arrived at by testing a pickup that had a severe history of doing this. It was discovered that if the brakes were released, the steering would return to normal. This wasn't a viable solution though cause sometimes turning and braking at the same time is desireable. After a round of new boosters, the problem went away for the most part. The aforementioned rig was involved in a fatality accident later because of a return of this same problem.