Interpretation ID: 77-3.7

TYPE: INTERPRETATION-NHTSA

DATE: 06/22/77

FROM: AUTHOR UNAVAILABLE; J. J. Levin, Jr.; NHTSA

TO: United Brake & Clutch

TITLE: FMVSS INTERPRETATION

TEXT: This responds to United Brake and Clutch's June 1, 1977, request for confirmation that use of a brake chamber equipped with separate diaphrams for application of service brake air pressure and isolated air pressure, along with a mechanical device that automatically holds the brakes in the applied position once they have been applied by means of the protected source of air, would comply with Standard No. 121, Air Brake Systems. The relevant provision of the standard states:

S5.6.3 Application and holding. The parking brakes shall be applied by an energy source that is not affected by loss of air pressure or brake fluid pressure in the service brake system. Once applied, the parking brakes shall be held in the applied position solely by mechanical means.

The National Highway Traffic Safety Administration is unable to "approve" system designs for compliance with a standard in advance, because there is no way to establish that a vehicle so equipped actually meets the requirements until it has been manufactured.

From your description, it appears that the design would not violate any provision of the parking brake requirement. Our understanding is that the protected source of air pressure is connected separately to the brake chamber, that it is designed to operate even with failure of the service brake chamber diaphram, and that the mechanical holding device operates automatically whenever air pressure in the trailer supply line is at atmospheric pressure. We assume also, that the braking force developed by the protected source of air pressure and maintained by the mechanical device would comply with the requirements of S5.6.1 or S5.6.2 of the standard.

Sincerely,

ATTACH.

UNITED BRAKE & CLUTCH

JUNE 1, 1977

JOE LEVIN -- CHIEF COUNSEL

RE: PARKLOCK BRAKING DEVICE

DEAR MR. LEVIN: IN DISCUSSIONS WITH MR. TED HERLIKY AND MR. DUANE PERRIN, IT WAS SUGGESTED THAT WE CONTACT YOUR OFFICE FOR THE PURPOSE OF OBTAINING A DEFINITION AS TO THE COMPLIANCE TO SS 121 OF OUR DEVICE.

THE PARKLOCK BRAKING DEVICE FUNCTIONS EXACTLY THE SAME AS THE BENDIX WESTINGHOUSE'S DD 3. THERE IS NO DIFFERENCE, FUNCTIONALLY. THE SAME PLUMBING ARRANGEMENT IS UTILIZED, THE BRAKE IS HELD IN APPLIED POSITION BY MECHANICAL MEANS. THE UNIT IS A DOUBLE DIAPHRAM UNIT, ONE FOR SERVICE AND THE OTHER FOR EMERGENCY OR PARKING. THE SOURCE OF ENERGY IN THE EMERGENCY OR PARKING APPLICATIONS, LIKE THE DD 3, IS A SEPARATE ISOLATED AIR SOURCE FROM THE SERVICE.

IF YOU CAN TELL ME IF THE UNIT I HAVE DESCRIBED WOULD COMPLY TO THE SS 121 LAW, FOR BOTH TRUCKS AND TRAILERS, IT WOULD BE MOST HELPFUL.

ENCLOSED YOU WILL FIND A DESCRIPTIVE BROCHURE ALONG WITH A BLOW UP OF YOUR PRODUCT.

THANK YOU VERY MUCH AND YOUR EXPEDIENT ATTENTION WILL BE GREATLY APPRECIATED.

YOURS TRULY,

RICHARD P. SEIB, PRES.

[Enc. Omitted]