Interpretation ID: aiam1512
Jr.
Senior General Attorney
Southern Railway System
Law Department
P.O. Box 1808
Washington
DC 20013;
Dear Mr. Stallsmith: In your letters of January 10, 1974, and May 17, 1974, you have aske if a trailer would comply with S5.8 of Standard No. 121, *Air brake systems*, ('have a parking brake system. . . when the air pressure in the supply line is at atmospheric pressure') if its parking brake system contained a valve that allows manual release of the parking brake with the supply line at atmospheric pressure but automatically resets itself when the supply line is pressurized, so that the parking brake system operates as specified by S5.8. Such a valve would permit limited motion of trailers on flat cars to cushion shock experienced during transit.; Such a valve does not appear to conflict with the intent of S5.8 tha the parking brake system apply when the supply line is at atmospheric pressure. The requirement is not intended to interfere with intentional manual release of the parking brakes after automatic application has occurred. The danger of inadvertent disablement of the parking brake system during subsequent highway travel is avoided by the automatic features which would return the system to normal operation as soon as the trailer is connected to a source of air pressure, i.e., a tractor.; Sincerely, Richard B. Dyson, Acting Chief Counsel