Interpretation ID: nht72-2.37

DATE: 07/21/72

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

TO: Motor Coach Industries, Inc.

TITLE: FMVSS INTERPRETATION

TEXT: This is in reply to your letter of April 17, 1972, in which you requested our interpretation of several sections of Standard 121. I apologize for our delay in replying.

Your first question concerns the air reservoirs which are considered to be included in the service reservoir system. As shown in your diagram, three tanks are capable of providing air to the service brake chambers: the wet air tank, the dry air tank, and the accessory tank. The first two tanks are clearly part of the service reservoir system. The accessory tank, however, has not been generally considered as part of the service brake system, and it is our opinion that it should not be included in computing the reservoir system capacity.

Although the exclusion of the accessory tank from the service reservoir system would seem to weigh against the present location of your pressure gauge in the accessory system circuit, there is another feature of its operation in its present location which leads us to conclude that it would not be acceptable under the present wording of S5.1.4. In the event of a pressure loss in the accessory system (your Condition #1), the gauge would accurately indicate the pressure in the dry tank until the pressure falls to 65 p.s.i., at which point the gauge would cease to indicate the dry tank pressure and would be only an accessory tank gauge. Thus, if the check valve functions properly, the dry tank would be at 65 p.s.i. even though the gauge may read O p.s.i. Because the pressure deliverable to the brake from the service reservoir system would be the 65 p.s.i. of the dry tank, the gauge would not be indicating the service reservoir system air pressure as required by S5.1.4.

Your third question is whether the vehicle must be stationary throughout the static retardation force test of S5.6.1. Our reply is that the vehicle need not remain stationary. Its friction may be overcome by the test pull, although it must exert a force of the magnitude specified in the section.

Your last question relates to the treatment of trailing axles under the requirements of S5.7.1. You indicate that you presently offer an automatic emergency system as an option and that it appears inconsistent to require park brakes on each axle under the automatic application option when they are not required on each axle the other option. We are continuing our evaluation of the parking brake requirements, including the axle-by-axle braking required by S5.7.1. At this time it has not been decided whether to formally institute rulemaking to adjust the requirements. We will advise you if such rulemaking will be forthcoming.