Interpretation ID: aiam3741
Manager
Safety & Emission Control Engineering
BMW of North America
Inc.
Montvale
New Jersey 07645;
Dear Mr. Ziwica: This responds to your letter requesting an interpretation of Standar No. 105, *Hydraulic Brake Systems*. Your request was in regard to a type of brake reservoir you are considering producing which would contain common fluid for the brake circuits and the brake hydraulic power assist unit. The issue raised by your letter was whether section S5.4.2 of the standard permits the common fluid to be counted as part of the minimum capacity required for the braking system. As discussed below, the answer to that question is no.; 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.<<<; As noted by your letter, the agency has previously interpreted thi section with respect to a brake reservoir servicing both the braking system and the clutch. In an October 9, 1981, letter to Toyota, we explained:; >>>This section specifies the total minimum fluid capacity that 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 minimu fluid capacity requirements 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 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 the clutch, none of that common fluid may be counted toward meeting the minimum requirements of section S5.4.2.<<<; This same rationale applies to a reservoir which contains common flui for the brake circuits and brake hydraulic power assist unit. As indicated in our October 1981 letter, the minimum fluid capacity requirements were determined by reference to the vehicle's braking system for the purpose of assuring that a vehicle's braking system reservoirs have adequate fluid capacity to service the brakes, i.e., the wheel cylinders and pistons. More specifically, the requirement for minimum capacity equivalent to the fluid displacement resulting when all the wheel cylinders of clipper pistons serviced by the reservoirs move from a new lining, fully retracted position to a fully worn, fully applied position, was based upon maintaining a sufficient supply of fluid to enable a vehicle to stop even when there was complete brake lining wear-out in the service brake. Put another way, the requirement assures an adequate supply of brake fluid over the lifetime of the brake linings, even if a driver fails to add fluid as part of routine maintenance.; This purpose would not be met if fluid available to both the brak circuits and brake hydraulic power assist unit was counted toward meeting the minimum fluid capacity requirements. Some of the common fluid would be used by the brake hydraulic power assist unit in normal service and thus be unavailable to the brake circuits. Moreover, in some instances of brake hydraulic power assist unit failure (e.g., a failure in the brake hydraulic power assist unit return line), all of the common fluid might be used by that unit.; Sincerely, Frank Berndt, Chief Counsel