Interpretation ID: nht88-3.61

TYPE: INTERPRETATION-NHTSA

DATE: 10/03/88 EST

FROM: ERIKA Z. JONES -- NHTSA CHIEF COUNSEL

TO: D. BURKARD; H. T. EBNER; ALFRED TEVES

ATTACHMT: OCTOBER 9, 1981 LETTER FROM BERNDT TO KAWANO, FEBRUARY 3, 1981 LETTER FROM KAWANO TO BERNDT, JULY 10, 1974 LETTER FROM DYSON TO NAKAJIMA, AND MAY 24, 1974 FROM TEVES TO GREGORY

TEXT: This responds to your letter concerning the brake fluid reservoir requirements of Federal Motor Vehicle Safety Standard No. 105, Hydraulic Brake Systems. You raised several issues concerning the standard in regard to a brake system you are considering p roducing. The issues raised by your letter are addressed below.

By way of background information, the National Highway Traffic Safety Administration does not provide approvals of motor vehicles or motor vehicle equipment. Under the National Traffic and Motor Vehicle Safety Act, it is the responsibility of the manufa cturer to ensure that its vehicles or equipment comply with applicable standards. The following represents our opinion based on the facts provided in your letter.

The master cylinder reservoir of your proposed brake system can be described as follows. The master cylinder services two brake circuits, Brake Circuit 1 and Brake Circuit 2, and an ancillary unit which itself services Brake Circuits 1 and 2 directly. The total volume (V) of the master cylinder equals V[1] + V[2] + V[3] + V[4], where:

V[1] represents the volume of an individual compartment (compartment V[1] servicing Brake Circuit 1. The fluid represented by V[1] is not available to Brake Circuit 2 or the ancillary unit.

V[2] refers to the volume of an individual compartment (compartment V[2] servicing Brake Circuit 2. The fluid represented by V[2] is not available to Brake Circuit 1 or the ancillary unit.

The sum of V[3] + V[4] refers to the volume of the portion of the reservoir which provides a common supply to Brake Circuits 1 and 2 and the ancillary unit. V[4] refers to that portion of this volume which is above the level where a fluid level indica tor lamp is activated.

You noted that your proposed brake fluid reservoir design differs from conventional designs in having the exit for the ancillary unit. You stated that you believe your design meets the requirements of sections S5.4.2 and S5.3.1(b), and requested confirm ation of your interpretation.

Section S5.4.2 reads as follows: S5.4.2 Reservoir capacity. 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 m ove 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. Reservoirs shall have completely separa te compartments for each subsystem except that in reservoir systems utilizing a portion of the reservoir for a common supply to two or more subsystems, individual partial compartments shall each have a minimum volume of fluid equal to at least the volume displaced by the master cylinder piston servicing the subsystem, during a full stroke of the piston. . . .

With respect to the requirement expressed in the first sentence of section S5.4.2, you stated that the total volume (V), i.e., the sum of V[1] + V[2] + V[3] + V[4], of your reservoir is "greater or equivalent to fluid displacement resulting when all whee l calipers move from a new lining position to a fully worn lining position." An issue raised by this statement is whether you are correctly calculating the total volume for purposes of section S5.4.2. More specifically, the issue is whether fluid which i s available for use by the ancillary unit, i.e., the fluid represented by V[3] and V[4], can be counted as part of the minimum capacity required by section S5.4.2.

It is our opinion that such fluid can be counted, since, even with the presence of the ancillary unit, the fluid is solely available to the brakes. In an October 9, 1981 interpretation to Toyota (copy enclosed), the agency interpreted section S5.4.2 to require that the minimum fluid capacity requirements be met by fluid which is solely available to the brakes. In that letter, the agency concluded that fluid which was available to both the brakes and the clutch could not be counted, since some or all o f the fluid might be used by the clutch and thus not be available for the brakes. The ancillary unit in your design is not comparable to the clutch, however, since it is part of the brake system and does not use brake fluid for purposes other than for t he brake circuits.

With respect to the requirement expressed in the second sentence of section S5.4.2, your letter indicates that V[1] and V[2] are each greater than or equivalent to the volume displaced by a full stroke of the related master cylinder piston. Given this s tatement, and the fact that compartments V[1] and V[2] are separate compartments such that their fluid is neither available to the other brake circuit or to the ancillary unit, we do not see any particular interpretation question raised by your design fo r this requirement.

For manufacturers choosing to meet Standard No. 105's brake system indicator lamp requirements by means of a fluid level indicator lamp, section S5.3.1(b) requires activation of the lamp under the following condition:

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.

With respect to this requirement, you stated that (V[1] + V[3]) is greater than or equivalent to one-fourth of (V[1] + V[3] + V[4]), and that (V[2] + V[3]) is greater than or equivalent to one-fourth of (V[2] + V[3] + V[4]). An issue raised by this stat ement is whether you are counting the correct fluid in determining the minimum warning level specified in section S5.3.1(b).

The issue of which fluid should be counted in determining the minimum warning level specified in section S5.3.1(b) was addressed in the letter to Toyota, discussed above. As discussed in that letter, the minimum warning level is determined by the fluid capacity of each compartment and not the capacity of the reservoir, unless the manufacturer recommends a higher safe level. In reference to your design, the compartments in question are compartment V[1] and compartment V[2]. Thus the warning level for compartment V[1] must not be less than 1/4 the capacity of compartment V[1]. Similarly, the warning level for compartment V[2] must not be less than 1/4 the capacity of compartment V[2]. This interpretation differs from your stated understanding, and ma y result in a lower minimum warning level. 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 your design.

Enclosure