499 S. Ridge Road
Lake Forest, IL 60045
Dear Mr. Ostrowski:
This responds to your faxed letter concerning Federal Motor Vehicle Safety Standard No. 105, Hydraulic Brake Systems. You asked us to "evaluate the brake system design of 1987-1990 Range Rovers" for compliance with that standard. More specifically, you provided diagrams of a brake system design for those vehicles and asked whether the design came within the standard's definition of "split service brake system."
By way of background information, the National Highway Traffic Safety Administration does not approve motor vehicles or motor vehicle equipment. Under 49 U.S.C. Chapter 301, it is the responsibility of the manufacturer to certify that its vehicles or equipment comply with applicable standards. Moreover, NHTSA cannot, outside the context of a compliance proceeding, make a determination of whether particular vehicles fail to comply with a Federal motor vehicle safety standard.
I can, however, discuss Standard No. 105's definition of "split service brake system" in the context of the diagrams you provided. As discussed below, the brake system for which you provided diagrams appears to come within that definition.
The term "split service brake system" is defined in S4 as "a brake system consisting of two or more subsystems actuated by a single control designed so that a leakage-type failure of a pressure component in a single subsystem (except structural failure of a housing to two or more subsystem) shall not impair the operation of any other subsystem."
As we understand the diagrams, the brake system at issue has two subsystems. The first subsystem, which we will refer to as Subsystem 1, is identified as the "primary hydraulic circuit." The second subsystem, which we will refer to as Subsystem 2, is identified as the "secondary hydraulic circuit."
Subsystem 2 includes the hydraulic lines and components exiting the master cylinder which activate the aft piston in each front
brake caliper, and one piston in each rear brake caliper. Subsystem 1 includes the hydraulic lines and components exiting the master cylinder which activate one forward piston in each front brake caliper.
Based on our understanding of the diagrams, a leakage-type failure in Subsystem 1 does not impair the operation of Subsystem 2, or vice-versa. Therefore, the brake system appears to come within the standard's definition of "split service brake system"
In your letter, you state that a leak in either rear hydraulic line impairs the front brakes, since a portion of front brake effectiveness is also lost. While this is correct, it does not mean that the brake system is not a split service brake system within the meaning of Standard No. 105, because the front and rear brakes are not the two "subsystems" for this brake system. I note that the standard does not require any particular kind of split service brake system, such as a front/rear system. (In a front/rear split service brake system, a leak in the rear system would not impair the front system.) The standard does, however, require vehicles to meet stopping distance requirements with a leakage failure in either subsystem. However, you have not provided any information indicating that the vehicles could not meet those stopping distance requirements with a failure in Subsystem 2.
You also raised a number of other concerns about the brake systems on 1987-1990 Range Rovers, including the number of owner complaints. We understand that our Offices of Vehicle Safety Compliance and Defects Investigation have been discussing those issues with you.
If you have further questions about this interpretation, please contact Edward Glancy of my staff at (202) 366-2992.
Acting Chief Counsel