49 CFR § 570.54 - Definitions.

---
identifier: "/us/cfr/t49/s570.54"
source: "ecfr"
legal_status: "authoritative_unofficial"
title: "49 CFR § 570.54 - Definitions."
title_number: 49
title_name: "Transportation"
section_number: "570.54"
section_name: "Definitions."
chapter_name: "NATIONAL HIGHWAY TRAFFIC SAFETY ADMINISTRATION, DEPARTMENT OF TRANSPORTATION"
part_number: "570"
part_name: "VEHICLE IN USE INSPECTION STANDARDS"
positive_law: false
currency: "2026-03-24"
last_updated: "2026-03-24"
format_version: "1.1.0"
generator: "[email protected]"
authority: "Secs. 103, 108, 119, Pub. L. 89-563, 80 Stat. 718 (15 U.S.C. 1392, 1397, 1407); delegation of authority at 49 CFR 1.50."
cfr_part: "570"
---


# 570.54 Definitions.

Unless otherwise indicated, all terms used in this part that are defined in part 571 of this chapter, Motor Vehicle Safety Standards, are used as defined in that part.

*Air-over-hydraulic brake subsystem* means a subsystem of the air brake that uses compressed air to transmit a force from the driver control to a hydraulic brake system to actuate the service brakes.

*Electric brake system* means a system that uses electric current to actuate the service brake.

*Vacuum brake system* means a system that uses a vacuum and atmospheric pressure for transmitting a force from the driver control to the service brake, but does not include a system that uses vacuum only to assist the driver in applying muscular force to hydraulic or mechanical components.