Mr. Michael Ostrowski
499 S. Ridge Road
Lake Forest, IL 60045
Dear Mr. Ostrowski:

This responds to your faxed letter concerning an interpretation we provided to you on August 27, 1996. I apologize for the delay in this response.

In your first letter, you asked us to "evaluate the brake system design of 1987-1990 Range Rovers" ("the I-H brake system") for compliance with Standard No. 105, Hydraulic Brake Systems. More specifically, you provided diagrams of a brake system design for those vehicles and asked whether the design came within Standard No. 105's definition of "split service brake system." We concluded that the brake system appeared to come within that definition.

In your follow-up letter, you raised a new issue concerning whether the Range Rover design comes within the definition of "split service brake system." As discussed below, the issue you raise does not change our prior conclusion.

Our earlier letter included the following discussion:

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 subsystems) 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 follow-up letter, you state that a retired brake engineer has advised you that "one key reason why the U.S. brake system suppliers and the Big 3 do not use the 'I-H' brake system is that a piston seal leak in the top piston of either front caliper will result in brake fluid wetting the brake pads on that wheel and therefore will 'impair the operation' of the other subsystem." You asked us to review again "the I-H arrangement of the subject 1987 and later Range Rovers."

We do not have any information concerning whether a particular leakage-type failure in one subsystem of the brake system at issue would result in brake fluid wetting the brake pads. However, even if it did, we would not consider it to impair the operation of the other subsystem. In such an instance, the other subsystem would operate no differently than if the brake pads were not wet. Of course, as we noted in our earlier letter, Standard No. 105 does require vehicles to meet stopping distance requirements with a leakage failure in either subsystem.

If you have further questions about this interpretation, please contact Edward Glancy of my staff at (202) 366-2992.

Sincerely,
John Womack
Acting Chief Counsel
ref:105
d.8/1/97