Interpretation ID: aiam2023
Director of Engineering
Peerless Division - Royal Industries
P.O. Box 445
Tualatin
OR 97062;
Dear Mr. Baker: This responds to your July 15, 1975, question whether Standard No. 121 *Air Brake Systems*, requires 'hold back valves' on air brake system reservoirs to guard against loss of air pressure through auxiliary equipment installations.; The answer to your question is no. Standard No. 121 does not contain prohibition on the use of air pressure from the air brake system for powering auxiliary devices. The vehicle must, of course, conform to Standard No. 121 following installation of the devices, if the installation occurs prior to the first purchase in good faith for purposes other than resale. For example, the compressor build-up pressure must still meet S5.1 of the standard whether or not auxiliary equipment is installed.; Although not a requirement of the standard, the NHTSA does consider i appropriate that a pressure protective valve be placed in the line to the auxiliary device so that a rupture of an auxiliary line does not cause depletion of air pressure in the brake system.; Sincerely, Frank A. Berndt, Acting Chief Counsel