Interpretation ID: aiam5049
Inc. 215 East Watkins Street Phoenix
AZ 85004;
"Dear Mr. Mario: This letter responds to your follow-up inquiry about recent amendment to Federal Motor Vehicle Safety Standard No. 121, Air Brake Systems that deletes the requirement for a trailer to have a separate reservoir capable of releasing the parking brake. The final rule allows air from the tractor supply lines to be used to release the parking brakes instead of air from such a reservoir. (56 FR 50666, October 8, 1991, copy enclosed) In our earlier response to you dated May 21, 1992, we stated that a trailer could be equipped with a protected separate reservoir after the amendment became effective. That letter explained that while the amendment deleted a provision requiring a protected service reservoir, nothing in the amendment would prohibit a trailer from being equipped with this device. You now ask what pressure must be retained in the supply line with any single leakage type failure in the service brake system if an original equipment manufacturer decides to use a system with a protected reservoir. You explained that you are aware of a system that has a 60 p.s.i. pressure amplification valve that results in having 90 p.s.i. holding off the spring brakes. I am pleased to have this opportunity to further explain our requirements to you. In deleting the provision requiring a separate reservoir capable of releasing the parking brake, the agency adopted requirements in S5.8 addressing supply line pressure retention. That provision states that under certain test conditions 'any single leakage type failure in the service brake system (except for a failure of the supply line, a valve directly connected to the supply line or a component of a brake chamber housing) shall not result in the pressure in the supply line falling below 70 p.s.i., measured at the forward trailer supply coupling. . .' (S5.8.2) Based on the above provision, the pressure in the supply line is not permitted to fall below 70 p.s.i when measured at the forward trailer supply coupling. Accordingly, a system with 60 p.s.i. in the supply line measured at the forward trailer supply coupling would not comply with the express requirements set forth in S5.8.2. Please be aware that after October 8, 1992, the provision in S5.8.2 applies to all new trailers, including those manufactured with a protected separate reservoir. Your letter appears to imply that there would be no safety problem in having the pressure in the supply line falling to 60 p.s.i. if a pressure amplification valve served to hold off the spring brakes. I note, however, that problems could occur if only one trailer in a mixed train, multiple trailer combination used such an amplification valve. For example, if a single leakage-type failure in the service brake system of such a trailer resulted in the supply line pressure falling to 60 p.s.i., this could cause the supply line pressure in following trailers to also fall to 60 p.s.i. If the following trailers were not equipped with an amplification valve, they could experience brake drag. I hope this information is helpful. If you have any further questions about NHTSA's safety standards, please feel free to contact Marvin Shaw of my staff at this address or by telephone at (202) 366-2992. Sincerely, Paul Jackson Rice Chief Counsel Enclosure ";