SUMMARY: RECENT ROCKWELL DEFECT REPORT IDENTIFIED THE FOLLOWING ADDITIONAL CONCERNS:A: INSUFFICIENT AIR PRESSURE DUE TO INADEQUATE AIR COMPRESSOR CAPACITY;B: AIR HORN PLUMBED INTO AIR SYSTEM WITHOUT PRESSURE PROTECTION VALVE;C: BRAKE IMBALANCE CAUSED BY PROPORTIONING VALVES WHICH REDUCE THE FRONT PRESSURE. THE ABOVE IDENTIFIED PROBLEMS MAY RESULT IN HEAT CHECKING AND CRACKING OF THE REAR BRAKE DRUMS.