One problem related to brake drum wheel cylinder has been reported for the 1996 GMC Jimmy. The most recently reported issues are listed below.
On two occasions brakes failed to stop vehicle. Both times vehicle was slowing down for complete stop when a loud high pitch sound came from the vehicle, and brake pedal went down to the floor; vehicle failed to stop. Vehicle was inspected on both occasions. First time, master cylinder was replaced, but it did not solve problem. Currently, vehicle is still in the repair shop. Seeking help from NHTSA.