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Every commercial motor vehicle (CMV) must be equipped with brakes acting on all wheels. The brakes must be adequate to stop and hold the vehicle or combination of vehicles.
Each commercial motor vehicle must have service, parking, and emergency brakes that meet the requirements of Part 393 Subpart C.
Scope
These requirements apply to CMVs operated in interstate commerce.
Regulatory citations
- 49 CFR 393 Subpart C — Brakes
Key definitions
- Brake: An energy conversion mechanism used to stop or hold a vehicle stationary.
- Commercial motor vehicle (CMV): A self-propelled or towed motor vehicle used on a highway, in interstate commerce, that meets any one of the following criteria:
- Has a gross vehicle weight rating (GVWR) or gross combination weight rating (GCWR), or gross vehicle weight (GVW) or gross combination weight (GCW), of 10,001 pounds or more, whichever is greater; or
- Is designed or used to transport more than 8 passengers including the driver) for compensation;
- Is designed or used to transport more than 15 passengers (including the driver), and not used to transport passengers for compensation; or
- Is transporting hazardous materials of a type or quantity which requires placarding.
- Emergency brake: A mechanism designed to stop a motor vehicle after a failure of the service brake system.
- Parking brake system: A mechanism designed to prevent movement of a stationary motor vehicle.
- Service brake system: A primary brake system used for slowing and stopping a vehicle.
Summary of requirements
The Federal Motor Carrier Safety Regulations (FMCSRs) require every CMV to be equipped with brakes "adequate to stop and hold the vehicle or combination of motor vehicles."
CMVs must be equipped with:
- Service brakes, whether hydraulic, air, vacuum, or electric;
- Parking brakes; and
- Emergency brakes.
All CMV brakes must be:
- Operable at all times, with limited exceptions.
- In compliance with the Federal Motor Vehicle Safety Standards (FMVSSs) in effect at the time of manufacture. The applicable standards include:
- FMVSS No. 105, Hydraulic and electric brake systems; and
- FMVSS No. 121, Air brake systems.
- Hydraulic and air brake systems that were not subject to these standards on the date of manufacture, and all vacuum and electric brake systems, must comply with Part 393 Subpart C.
- Acting on all wheels. Under 393.42(b), there are exceptions for:
- Certain trucks or truck tractors manufactured before July 25, 1980;
- Certain vehicles being towed in a driveaway-towaway operation;
- Certain trailers with a gross weight of 3,000 pounds or less;
- The steering axle of a three-axle dolly steered by a co-driver; and
- Certain heavy-haul trailers that operate at 20 mph or less.
- Equipped with a working antilock braking system (ABS). ABS is required as of:
- March 1, 1997, for air brake-equipped tractors;
- March 1, 1998, for all other air brake-equipped commercial vehicles (including trailers); and
- March 1, 1999 for CMVs with hydraulic brakes.
- As well as having an ABS system, these vehicles must also have functioning malfunctioning warning lights, to alert the driver to a malfunction in the ABS. The requirements on the warning lights vary, depending on vehicle configuration and year of manufacture (see 393.55 for details).
Trucks and truck tractors must be fitted with a warning signal that will alert the driver to a failure in the service brake system.
A single valve must apply all the service brakes on the vehicle or combination of vehicles, except in driveaway-towaway operations. (Note, however, that this does not prohibit a vehicle from being equipped with an additional valve to operate the trailer brakes.)
All slack adjusters on air-brake vehicles built after on or after October 20, 1994, and all vehicles with hydraulic brakes built after October 20, 1993, must be able to automatically adjust for wear that normally occurs in the brake system. These are referred to as “automatic slack adjusters,” or simply “ASAs.”
Parking brakes. The parking brake system must:
- Be adequate to hold the vehicle or combination of vehicles on any grade on which it is operated, under any condition of loading in which it is found on a public road (free of ice and snow).
- Be capable of being applied by either the driver's muscular effort or by spring action, except for air-applied, mechanically-held parking brake systems which meet the parking brake requirements of FMVSS No. 121.
- Be held in the applied position by energy other than fluid pressure, air pressure, or electric energy. The parking brake system must not be capable of being released unless adequate energy is available to immediately reapply the parking brake with the required effectiveness.
Emergency and breakaway brakes (air brakes). If a motor vehicle is used to tow a trailer equipped with brakes, then the service brakes on the towing vehicle must be able to stop the vehicle if the trailer breaks away. For air-braked towing units, the tractor protection valve or similar device must operate automatically when the air pressure on the towing vehicle is between 20 psi and 45 psi.
Every truck or truck tractor with air brakes, when used for towing other air-braked vehicles (except in driveaway-towaway operations), must have two means of activating the emergency trailer brakes:
- One that operates automatically if the towing vehicle's air supply decreases to a fixed pressure of between 20 psi and 45 psi.
- One that is controlled manually from the driving seat, with a clear indication of its emergency position or method of operation. Use of this manual method cannot prevent operation of the automatic method.
The automatic and manual methods may be, but are not required to be, separate.
Emergency and breakaway brakes (vacuum brakes). Every truck tractor and truck when used for towing other vehicles equipped with vacuum brakes (except in driveaway-towaway operations) must have (in addition to the single main-brake control) a manual control device which can be used to operate the brakes on the towed vehicles in emergencies. This second control must be independent of:
- Brake air, hydraulic, and other pressure (unless pressure failure causes the towed vehicle brakes to be applied automatically); and
- Other controls.
Trailer breakaway. Trailer brakes, when required, must apply on at least two axles automatically and immediately upon breakaway from the towing vehicle, and must remain applied for 15 minutes.
Emergency valves. Air-brake systems on towed vehicles must have "no bleed-back" valves or equivalent devices to prevent backflow of air to the towing vehicle.