Interpretation ID: nht74-2.17

DATE: 08/12/74

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

TO: Walter Case

TITLE: FMVSS INTERPRETATION

TEXT: This responds to your June 13, 1974, request for approval of your "park-lock" device under the parking brake system requirements of Standard No. 121, Air brake systems, 49 CFR @ 571.121. After a trailer's emergency air supply applies the brakes through the service brake chamber in response to a low service brake air supply, your device locks the brakes in the applied position.

The relevant provision of the standard states:

S5.6.3 Application and holding. The parking brakes shall be applied by an energy source that is not affected by loss of air pressure or brake fluid pressure in the service brake system. Once applied, the parking brakes shall be held in the applied position solely by mechanical means.

The arrangement described would not meet this requirement because the energy source to apply the brakes (the emergency air supply) would be affected by loss of air pressure in the service brake system. For example, any failure in the service brake piston diaphragm would cause a loss of air pressure that would in turn "affect" the energy source that applies the parking brake. The brake chamber housing assembly is an element which is not considered to be part of the service brake system for this requirement.

Standard No. 121 does not specify the design of brake system components. Therefore we neither approve nor disapprove the use of particular components like the "park-lock" device. It may be used in any parking brake system which meets the requirements of the standard.

I would like to point out that the standard becomes effective January 1, 1975, for trailers, and that it does not regulate air brake systems on trailers manufactured before that date.

Yours truly,

ATTACH.

June 13, 1974

Larry Schneider -- Chief Counsel National Highway Admn.

Dear Mr. Schneider:

This is being written at the suggestion of Mr. Sid Williams, made to me during a telephone conversation only a few minutes ago.

I am the inventor of a brake locking device for air brake system equipped trailers. This device which weighs barely two pounds has been pronounced by leaders of the Motor Freight industry as the greatest safety device offered the industry in thirty-five years.

Some eight or so weeks ago, B. F. Goodrich Company examined and tested this brake lock and informed us of their intention to use it on their new safety systems, these being readied for market. This followed careful examination of the requirements for complying with NVSS 121 by both B. F. Goodrich and ourselves.

Our device does not alter the regular braking system in any manner. This brake lock works in conunction with the long required and approved Reserve Emergency Valve and trailer emergency air supply system. With the improved (modulated) RE 4 Valve plus our (Park-Lock) brake holding device, safety involving trailers has been increased many fold whether on the highway, parking lot or at the loading dock.

Should the service brake air supply be broken intentionally or by accident the trailer emergency air supply would take over and bring the vehicle to a stop. In such an instance, the Park-Lock would lock the set brakes automatically when the air pressure dropped to 20 lbs.

We submit that our brake locking device meets all requirements of NVSS 121 and of particular reference to (e) paragraph of Div. 12-369-26508 which prompted B. F. Goodrich's bringing to our attention they had been informed there was some question about Park-Lock meeting one requirement. The question concerns a brake chamber diaphragm failure.

Conversations with two large Motor Freight Companies (Transcon - E.T.M.F.) answered our inquiry this way: Concerning failure or malfunction of a diaphragm was, according to their records the rarest of their problems. The only maintenance reports are that of slow leaks reported by the driver. Neither Company could recall a burst diaphragm. In the event of such leak there is a warning device in the cab calling such to the driver's attention.

Particular care was given to explain to us that should the service system fail, the trailers system would be activated by the RE 4 Valve and the vehicles would be brought to a stop, even with a leak in a diaphragm.

Should the trailer system malfunction, the tractor system, protected by the tractor protection valve, would provide the necessary braking power to bring the vehicles to a safe stop. Either way, they emphasize, adequate braking is there to bring vehicles to a stop and all that is really needed for safety is a satisfactory brake lock to fit the tried and proven system now in use.

The Park-Lock brake lock has been proven in regular service for over a year. It is small enough to hold in your hand. The device is simple of construction, has three moving parts making for little wear, or requirement of maintenance.

We will appreciate your careful examination and checking out of our claims. The same, we are confident, can be said for the industry who has hailed our safety device.

Sincerely yours, Walter Case -- PARK-LOCK, INC.