• All wheels on a CMVs must be equipped with operative brakes adequate to stop and hold the vehicle or combination of motor vehicles.
• Each CMV must meet the applicable service, parking, and emergency brake requirements.
• The requirement that a CMV be equipped with operative brakes on all wheels also applies to any motor vehicle towed by means of a tow-bar when another motor vehicle is full-mounted on the towed vehicle, and any saddlemount configuration with a fullmount.

One of the essential parts necessary for the safe operation of a commercial motor vehicle (CMV) under the Federal Motor Carrier Safety Regulations (FMCSRs) is brakes.

Required brake systems

Every CMV must have operative brakes adequate to stop and hold the vehicle or combination of motor vehicles. Each CMV must meet the applicable service, parking, and emergency brake requirements. (393.40; 393.48).

Service brake systems — The service brakes requirements, found in 393.40(b), include rules for the following braking systems:

• Hydraulic
• Air
• Vacuum
• Electric
• Surge

Parking brake systems — Each CMV must be equipped with a parking brake system that meets the applicable requirements of 393.41. (393.40(c))

Emergency brakes — The emergency brake regulations, found in 393.40(d), are broken down into two categories: (1) partial failure of service brakes, and (2) vehicles manufactured on or after July 1, 1973.

• Partial failure of service brakes:
  • Hydraulic brake systems — Motor vehicles manufactured on or after September 2, 1983, and equipped with a split service brake system must, at a minimum, meet the partial failure requirements of Federal Motor Vehicle Safety Standard (FMVSS) No. 105 in effect on the date of manufacture.
  • Air brake systems — Buses, trucks and truck tractors manufactured on or after March 1, 1975, and trailers manufactured on or after January 1, 1975, must be equipped with an emergency brake system which, at a minimum, meets the requirements of FMVSS No. 121 in effect on the date of manufacture.
  • Vehicles not subject to FMVSS Nos. 105 and 121 on the date of manufacture — Buses, trucks and truck tractors not subject to FMVSS Nos. 105 or 121 on the date of manufacture must meet the requirements of 393.40(e). Trailers not subject to FMVSS No. 121 at the time of manufacture must meet the requirements of 393.43.
• Vehicles manufactured on or after July 1, 1973:
  • A bus, truck, truck tractor, or a combination of motor vehicles manufactured on or after July 1, 1973, and not a hydraulic or air brake system, must have an emergency brake system which consists of emergency features of the service brake system or an emergency system separate from the service brake system.
  • A control by which the driver applies the emergency brake system must be located so that the driver can operate it from the normal seating position while restrained by any seat belts with which the vehicle is equipped. The emergency brake control may be combined with either the service brake control or the parking brake control, but all three controls may not be combined.

Brakes required on all wheels

Every CMV must be equipped with brakes acting on all wheels. This requirement also applies to:

1. Any motor vehicle towed by means of a tow-bar when another motor vehicle is full-mounted on the towed vehicle; and
2. Any saddlemount configuration with a fullmount. (393.42)