Interpretation ID: aiam1572
10426 Desdemona Drive
Dallas
TX 75228;
Dear Mr. Case: This responds to your June 13, 1974, request for approval of you 'park-lock' device under the parking brake system requirements of Standard No. 121 *Air brake systems*, 49 CFR S571.121. After a trailer's emergency air supply applies the brakes through the service brake chamber in response to a low service brake air supply, your device locks the brakes in the applied position.; 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 arrangement described would not meet this requirement because th energy source to apply the brakes (the emergency air supply) would be affected by loss of air pressure in the service brake system. For example, any failure in the service brake piston diaphragm would cause a loss of air pressure that would in turn 'affect' the energy source that applies the parking brake. The brake chamber housing assembly is an element which is not considered to be part of the service brake system for this requirement.; Standard No. 121 does not specify the design of brake syste components. Therefore we neither approve nor disapprove the use of particular components like the 'park-lock' device. It may be used in any parking brake system which meets the requirements of the standard.; I would like to point out that the standard becomes effective Januar 1, 1975, for trailers, and that it does not regulate air brake systems on trailers manufactured before that date.; Yours truly, Richard B. Dyson, Acting Chief Counsel