My 1992 chevy silverado ext cab 2 wheel drive pickup, 106000 miles, 350 cu. In. Motor, brake trouble light comes on at exactly 37 mph. The brakes themselves are in perfect condition as is the emergency brake system. The owner's manual mentions that the vehicle has anti-lock brakes on the rear wheels. Where are the speed sensors located for this system? Could they be the source? Everything else has been gone over by a certified mechanic. Thanks.