Interpretation ID: aiam1890
Power Controls Division
Midland-Ross Corporation
490 South Chestnut Street
Owosso
MI 48867;
Dear Mr. Denholm: This responds to Midland-Ross' March 19, 1975, questions whether S5.7. of Standard No. 121, *Air brake systems*, (as effective September 1, 1976) specifies reservoirs that are charged to 100 psi before or after introduction of a failure as specified in S5.7.1, static or dynamic testing of emergency brake system application and release, and design limits of 1 to 60 psi for emergency brake system application and release. You also asked whether S5.7.4(c) requires modulation of the towed vehicle emergency system in cases of control line failure, and whether such a failure qualifies as a 'single failure in the service brake system' for purposes of emergency brake system performance under S5.7.1.; Section S5.7.3 specifies emergency application and release capabilit with all air reservoirs charged to 100 psi, followed by introduction of a failure. The vehicle is tested for this application and release capability statically. In our reconsideration of this amendment, NHTSA will consider a clarification of this language.; The maximum of 1 psi for pressure release and minimum 60 psi fo pressure application are intended only as objective measures of what constitute an application or a release. In the petitions for reconsideration of this amendment, it has been suggested that the ability to move the vehicle and then stop it after an emergency brake application would permit greater design freedom in the design of emergency brake systems. This issue will be addressed in our response to petitions for reconsideration.; Section S5.7.4(c) requires that a towing vehicle be capable o modulating the air in the supply or control line following a single failure in the service brake system on the towing vehicle, but does not require modulation of the towed vehicle emergency brake system under any circumstance (including control line failure).; This language is intended to assure that a single failure in the truc itself will not prevent modulation of an unimpaired system from the tractor protection system rearwards. A clarification of this language may be necessary.; Yours truly, Richard B. Dyson, Assistant Chief Counsel