Interpretation ID: nht80-1.37

DATE: 03/18/80

FROM: AUTHOR UNAVAILABLE; F. Berndt; NHTSA

TO: Aeroquip Corporation

TITLE: FMVSS INTERPRETATION

TEXT: This responds to your January 17, 1980, letter asking whether the 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 stated 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 performance of the parking brake system.

With respect to your question, you state that a piston does not fail in 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,

AEROQUIP CORPORATION JACKSON, MICHIGAN 49203

January 17, 1980

Roger Tilton Office of Chief Counsel National Highway Traffic Safety Administration

Re: (Docket No. 75-16; Notice 26) FR Vol. 44, No. 155, page 46850 Thursday, August 9, 1979

Dear Mr. Tilton:

The following statement in regard to the performance of emergency-parking brakes applied by air pressure rather than by spring force appears in the subject Federal Register:

"Thus, the prescribed performance must be achieved with any type of failure in the service brake system, including a ruptured diaphragm".

This statement makes it plain that the service and emergency-parking brake systems may not share a common diaphragm.

Since it is not dealt with in the ruling and because a piston brake does not fail in the complete and usually sudden manner of a diaphragm, we are interpreting the ruling to allow the use of a common piston, assuming all requirements of the ruling are met.

Would you please confirm our assumption at your earliest convenience. You may contact me at (517) 787-8121. Thank you for your cooperation.

C. Crissy Manager, Mechanical Products