Interpretation ID: aiam2182
Wagner Electric Corporation
Wagner Division
11444 Lackland Road
St. Louis
MO 63141;
Dear Mr. Kourik: This responds to Wagner Electric Corporation's October 21, 1975 question whether a trailer would satisfy the requirements of S5.2.1.1 of Standard No. 121, *Air Brake Systems*, to provide a reservoir 'that is unaffected by a loss of air pressure in the service brake system,' if the reservoir provided is either of two service brake system reservoirs on the vehicle, equipped with a pressure protection valve directly adjacent to each reservoir. The drawings enclosed in your letter indicate that the 'protected tank' that is normally provided, separate from the service brake system, would be eliminated and either of the service brake system reservoirs would be used to satisfy S5.2.1.1 in the event of a parking brake application.; Your interpretation of S5.2.1.1 is correct. That section calls for reservoir of air as an energy source that is used to release the vehicle's parking brakes after an automatic or manual application. In requiring that this reservoir be 'unaffected by a loss of air pressure in the service brake system,' the NHTSA means that a single failure of the service brake system would not result in loss of this air supply. With the pressure protection valves located as described in your enclosures, it appears that the system would comply with Section S5.2.1.1.; This 'single failure' requirement must be distinguished from th requirement of S5.6.3 that the energy source for application of the parking brake be 'not affected by loss of air pressure or brake fluid pressure in the service brake system.' The NHTSA has interpreted this requirement to require an uninterrupted energy source despite loss of all air pressure from the service brake system. We recognize that the language of the two passages is substantially identical, and should be changed for clarity.; In a recent proposal to revise the parking brake requirements of th standard (40 FR 56920, December 5, 1975), the NHTSA inadvertently failed to make this distinction clear in its newly-proposed definition of 'parking brake system' and intends to publish a correction of the proposal in the near future.; Yours truly, Richard B. Dyson, Assistant Chief Counsel