Interpretation ID: aiam3224
Manager
Mechanical Products
Aeroquip Corporation
300 South East Avenue
Jackson
MI 49203;
Dear Mr. Crissy: This responds to your Janaury 17, 1980, letter asking whether th performance requirements of the parking brake sections of Standard No. 121, *Air Brake Systems*, permit the use of a common piston for both the service and parking brakes.; In our August 9, 1979, Federal Register notice (44 FR 46850), we state that the performance requirements of the parking brake system must be achieved with any single leakage- type failure in the service brake system, including a ruptured diaphragm. The use of the diaphragm example was intended only to clarify a question that had been raised by a commenter to the notice proposing the parking brake amendment. The diaphragm example does not limit the requirement that any single leakage- type failure of a component of the service brake system must not affect the peformance of the parking brake system.; With respect to your question, you state that a piston does not fail i the sudden manner of a diaphragm. Although this may be accurate, it is not the correct approach for interpretation of the performance requirements of the parking brake sections of the standard. If your parking brake system would comply with the requirements of the standard once the piston in the service brake system has failed, then you would be permitted to use a common piston. If, on the other hand, a failure of the service brake piston would cause the parking brake system to fail the requirements, a common piston would not be allowed.; Sincerely, Frank Berndt, Chief Counsel