Interpretation ID: 77-4.17
TYPE: INTERPRETATION-NHTSA
DATE: 10/17/77
FROM: AUTHOR UNAVAILABLE; J. J. Levin, Jr.; NHTSA
TO: Midland-Ross Corporation
TITLE: FMVSS INTERPRETATION
TEXT: This responds to Midland-Ross' July 8, 1977, request for confirmation that the requirement that "[each] service reservoir system shall be protected against loss of air pressure. . .by check valves or equivalent devices" in S5.1.2.3 and S5.2.1.5 of Standard No. 121, Air Brake Systems, permits small losses of air pressure through the check valve of up to 2 psi per minute without constituting noncompliance.
The requirement for protection against "loss of air pressure" does permit a small amount of leakage, in recognition of the fact that no fitting can be perfectly air tight. While the standard does not presently specify a rate of permissible air loss in S5.1.2.3 or S5.2.1.5, the agency has adopted a maximum loss of 10 psi in 10 minutes as meeting the requirement for protection against loss of air pressure. The agency is considering adding such a specification to the standard in the future by interpretive amendment.
SINCERELY,
MIDLANDROSS CORPORATION
July 8, 1977
Thomas W. Herlihy Office of Chief Council National Highway Traffic Safety Adm.
Subject: FMVSS #571.121, Section S5.1.2.3.
Midland-Ross Corporation is a manufacturer of air brake actuation equipment for heavy duty trucks. We therefore manufacture components which are used by truck manufacturers for compliance with FMVSS #571.121. We are asking for interpretation of the requirement for check valves, specifically as it relates to section S.5.1.2.3.
Section S.5.1.2.3 states: "Each service reservoir system shall be protected against loss of air pressure due to failure or leakage in the system between the service reservoir and the source of air pressure, by check valves or equivalent devices whose proper functioning can be checked without disconnecting any air line or fitting".
The statement "loss of air pressure" has not been qualified and can be interpretated to mean zero leakage. It is common knowledge that check valves being produced today have a leakage tolerance. For example, Midland-Ross 100% inspects check valves and allows a maximum leakage of 330 ccm (cubic centimeters/minute) leakage at 20 psi. It is also common knowledge that some leakage will exist in connections upstream to the check valve through air lines, fittings and valves. In fact the accepted industry standards have been to allow a maximum leakage of 2 psi pressure drop in one minute (reference: SAE J-890, California Highway Patrol; DOT part 570, Inspection Standard for Motor Vehicles).