Interpretation ID: nht81-3.25

DATE: 10/09/81 EST

FROM: FRANK BERNDT -- CHIEF COUNSEL-NHTSA

TO: J. KAWANO -- GENERAL MANAGER, U.S. REPRESENTATIVE OFFICE - TOYOTA MOTOR CO., LTD.

TITLE: NOA-30 NONE

ATTACHMT: OCTOBER 3, 1988 LETTER FROM JONES TO BURKARD, EBNER, AND TEVES, FEBRUARY 3, 1981 LETTER FROM KAWANO TO BERNDT, JULY 10, 1974 LETTER FROM DYSON TO NAKAJIMA, MAY 24, 1974 LETTER FROM TEVES TO GREGORY, AND MAY 27, 1988 LETTER FROM TEVES TO JONES

TEXT: This responds to your letter of February 3, 1981, asking two questions about Safety Standard No. 105, Hydraulic Brake Systems. The questions were asked in regard to a type of brake reservoir you are considering producing which would service both the braking system and the clutch.

The first question is whether an interpretation you have made of section S5.4.2 of the standard is correct. The essential issue to that question is whether hydraulic fluid which is available to the clutch, either for normal use of in the event of clutch failure, can be counted as part of the minimum capacity required by section S5.4.2 for the braking system. The agency interprets the standard to require that the minimum fluid capacity requirements for brake reservoirs be met by fluid which is not available to the clutch, either during normal use or in the event of clutch failure. Thus, as will be explained below, your interpretation is incorrect.

Safety Standard No. 105 establishes requirements for a vehicle's braking system, including minimum capacity requirements for the reservoirs. Neither this standard nor any other Federal motor vehicle safety standard includes requirements for the reservoirs of a vehicle's clutch. While nothing prohibits a manufacturer from producing a master cylinder which services both the vehicle's braking system and clutch, the minimum fluid capacity requirements for reservoirs must be met separated for a vehicle's braking system.

The first sentence of section S5.4.2 states:

Reservoirs, whether for master cylinders or other type systems, shall have a total minimum capacity equivalent to the fluid displacement resulting when all the wheel cylinders or caliper pistons serviced by the reservoirs

move from a new lining, fully retracted position (as adjusted initially to the manufacturer's recommended setting) to a fully worn, fully applied position, as determined in accordance with S7.18(c) of this standard.

This section specifies the total minimum fluid capacity that a vehicle's braking system reservoirs must have. That amount is determined by reference to the vehicle's braking system, i.e., by the fluid displacement which results when all the wheel cylinders or caliper pistons serviced by the reservoirs move from a new lining, fully retracted position to a fully worn, fully applied position.

The purpose of this requirement is to assure that a vehicle's braking system reservoirs have adequate fluid capacity to service the brakes.

The agency interprets section S5.4.2 to require that the minimum fluid capacity requirements for brake reservoirs be met by fluid which is solely available to the brakes. If fluid is available to both the brakes and the clutch, some of that fluid will be used by the clutch in normal service and thus be unavailable to the brakes. In the event of clutch failure, all of the common fluid may be used by the clutch. Therefore, while Standard No. 105 does not prohibit manufacturers from producing master cylinders with reservoirs that have some fluid available to both the braking system and clutch, none of that common fluid may be counted toward meeting the minimum fluid capacity requirements of section S5.4.2.

Your interpretation of section S5.4.2 is incorrect because it counts common fluid toward meeting these requirements. In reference to Figure 1 that you attached with your letter, no fluid above the top of the wall dividing X and Y from Z should be counted toward meeting section S5.4.2's requirements.

We are aware that this interpretation conflicts with our July 10, 1974, interpretation that you attached with your letter and regret any inconvenience. That interpretation indicated that all five designs included in your letter of May 24, 1974, appeared to conform to section S5.4.1, providing that the reservoir capacity requirements of section S5.4.2 were met. That interpretation did not properly consider the requirements of section S5.4.2, as discussed above. It should have indicated that designs (3) and (4) do not comply with Standard No. 105 because they cannot meet section S5.4.2's minimum capacity requirements without counting fluid that is common to the braking system and clutch.

Your letter also asked about which fluid should be counted in determining the minimum warning level specified in section S5.3.1(b). That section refers to a drop in the level of brake fluid in any master cylinder reservoir compartment to less than the recommended safe level specified by the manufacturer or to one-fourth of the fluid capacity of that reservoir compartment, whichever is greater.

The minimum warning level is thus determined by the fluid capacity of each compartment rather than by the total capacity of the reservoir, unless the manufacturer recommendes a higher safe level. In reference to Figure 1 that you attached with your letter, the compartments in question are X and Y. Thus, the warning level for compartment X must not be less than 1/4 of the capacity of X. Similarly, the warning level for compartment Y must not be less than 1/4 the capacity of Y. Since there may be safety advantages to higher warning levels, particularly where the capacity of individual compartments is small in relation to the capacity of the reservoir, you may wish to specify a higher warning level such as that indicated in Figure 1, i.e., at a level above the wall separating X from Y.