Interpretation ID: nht75-1.13

DATE: 05/16/75

FROM: AUTHOR UNAVAILABLE; R. B. Dyson; NHTSA

TO: Toyo Kogyo Co., Ltd.

TITLE: FMVSS INTERPRETATION

TEXT: This responds to your April 16, 1975, question whether the test procedure of S7.7.1 of Standard No. 105-75, Hydraulic brake systems, permits reapplication of the parking brake if the vehicle fails to hold on the required grade after the first application of a force or series of forces. You also ask whether a brake warning indicator which signals a short-lived loss of pressure during a spike stop brake application would conform to S5.3.1(a)(1).

Section S7.7.1 directs (in part) application of the parking brake with a force or series of forces, release of the service brake which has been holding the vehicle on the required grade, and indicates that in release of the service brake, "it may be necessary to reapply it, if the vehicle moves slightly, to take up the parking brake system slack." (emphasis added).

The word "it" refers to the service brake system, and not the parking brake system. This sentence permits application of the service brake only, which has the effect of taking up parking brake system slack due to rotation of the brake shoes and drum prior to bottoming against the anchor pin. This service brake application is intended to provide the best opportunity for a static test of the parking brake.

You state that a differential pressure can occur within the Mazda master cylinder during a spike stop brake application because the piston travels beyond the outlet port to the rear wheel brake lines. This pressure differential causes momentary activation of the brake warning indicator lamp.

The NHTSA would consider in this case that, as a technicality, momentary failure of the rear wheel subsystem has occurred because continued braking pressure cannot be applied to the rear wheels. You point out that the system corrects immediately and the signal lamp is extinguished.

From your description of the Mazda system, NHTSA concludes that the activation of the signal lamp conforms to the requirements of S5.3.1(a)(1) as long as it is designed to extinguish as soon as the system corrects and continued brake force could be applied to the rear wheels.

Yours truly,

Enclosure

April 16, 1975

James C. Schultz -- Chief Counsel, National Highway Traffic Safety Admin.

Dear Mr. Schultz: In determining the final system for meeting MVSS 105-75, we are having some trouble in following some of the test requirements. We would highly appreciate it if we could have your view on the following questions until April 25, 1975 in order to relief them.

1. Re: Parking Brake Test Procedure in S7.7.1

When we run the test according to this section, we understand the following sequence (a) through (h) acceptable.

a) Bring the vehicle on the grade and hold it by applying the service brake.

b) Apply the parking brake by keeping the service brake working with specified force.

c) Release the service brake.

d) Check the movement.

e) If moved slightly, reapply the service brake and hold it.

f) Then reapply parking brake with the specified force to take up its slack without releasing it.

g) Release service brake again.

h) Check the movement.

2. Re: Brake Indicator Lamp

During the spike stop test on our model, it was observed that the "brake indicator lamp" lit while the test driver kept depressing the service brake pedal. We studied this problem and found that as shown in the drawing its master cylinder piston can travel beyond the hole which supplies brake fluid to the rear brake cylinder, that generates the difference in the pressure of front and rear.

As the brake indicator lamp works by sensing the difference in pressure of the front and rear, this lamp becomes "on" accordingly.

(Graphics omitted)

However, this piston does not travel beyond its hole under normal operation. If this lamp should light during spike stop, this will not give any problem to the drivers because this lamp will turn out after releasing pedal force.

We would like to have your view on the acceptability of the above condition.

Sincerely yours,

H. (Speedy) Hirai -- Technical Representative, Toyo Kogyo Co., Ltd. (Mazda), Representative office

cc: V. Bloom; T. Herliny