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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.
These requirements apply to CMVs operated in interstate commerce.
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:
All CMV brakes must be:
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:
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:
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:
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.