Interpretation ID: aiam2600
President
United Brake & Clutch
2100 thru 2116 South Ervay Street
Dallas
TX 75215;
Dear Mr. Seib: This responds to United Brake and Clutch's June 1, 1977, request fo confirmation that use of a brake chamber equipped with separate diaphrams (sic) 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 b 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 t '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 an 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 (sic), 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, Joseph J. Levin, Jr., Chief Counsel