Interpretation ID: aiam2525
Vice President and General Manager
The Berg Manufacturing Company
Iola
KS 66749;
Dear Mr. Campanini: This responds to Berg Manufacturing Company's February 9, 1977, reques for confirmation that the emergency and parking brake requirements for trailers contained in Standard No. 121, *Air Brake Systems*, would be satisfied by use of an emergency relay valve (ERV) that, upon a single failure in the service brake system upstream from the ERV, automatically directs the application of service brake air through the antilock valve to service brake chambers in proportion to the amount of air loss being experienced. While parking brake force is supplied first by service brake air and subsequently by means of spring brakes, the spring brake force is not utilized until service brake air pressure is lost in the service brake chamber. This occurs because the spring brakes are held off directly by service reservoir air pressure without provision of an isolated reservoir, controlled by a separate valve that directs spring brake application in response to loss of air pressure in the supply line.; From your description, it does not appear that the system complies wit S5.2.1.1 of Standard No. 121. Section S5.2.1.1 specifies that a reservoir shall be provided on a trailer that is capable of releasing the vehicle's parking brakes at least once and that is 'unaffected by loss of air pressure in the service brake system.' This requirement has been interpreted to mean that a single failure of the service brake system would not result in loss of the isolated air supply. A copy of this interpretation is enclosed for your information. It appears that a single failure of the service brake reservoir in the system you describe would result in the loss of the isolated air required by S5.2.1.1.; In other respects the system you describe does not appear to violat the requirements of Standard No. 121. The use of service air pressure to actuate the parking brakes has been used in certain bus applications and is permissible as long as a source of energy to apply the parking brakes is usable at all times and is unaffected by any single failure in the service brake system.; Sincerely, Frank Berndt, Acting Chief Counsel