Federal Motor Vehicle Safety Standards; Air Brake Systems; Air Compressor Cut-In

In response to a petition for reconsideration submitted by Flxible Corporation, this document amends Standard No. 121, Air Brake Systems, with respect to the air pressure at which a bus's air compressor must automatically activate. A bus manufacturer will be allowed to set the air compressor governor cut-in pressure at 85 psi or greater. The agency believes that allowing the air pressure to fall to 85 psi or greater, instead of 100 psi or greater, before the air compressor is required to cut in, provides a more appropriate activation pressure that accounts for the severe duty cycle experienced by some buses. By reducing the frequency of compressor operation, this modification will reduce potential safety problems caused by the air compressor introducing engine oil into the vehicle's air system.