Interpretation ID: aiam1835
Volvo of America Corp.
Rockleigh
NJ 07647;
Dear Mr. Taylor: This responds to Volvo of America Corporations's (sic) March 3, 1975 question whether a braking system which employs air pressure modulated by the vehicle operator to provide the energy used to actuate the brakes is an air brake system subject to Standard No. 121, *Air brake systems*.; The answer to this question is yes. We do not agree that such a syste could qualify as a hydraulic brake system under the definition found in Standard No. 105-75, *Hydraulic brake systems*. That definition requires that a hydraulic brake system employ hydraulic fluid as a medium for transmitting force 'from a service brake control to the service brake', which is not the case in the system you describe. The system in question appears to be what is popularly known as an 'air over hydraulic' system. With regard to such systems, NHTSA has stated in a preamble to the air brake standard: 'It should be noted that the term 'air brake system' as defined in this standard applies to the brake configuration commonly referred to as 'air over hydraulic,' in which failure of either medium can result in complete loss of braking ability.' (36 FR 3817, February 27, 1971).; Yours truly, Richard B. Dyson, Assistant Chief Counsel