CDL brakes study guide
Subpart C of FMCSA's parts and accessories rules (49 CFR 393.40 to 393.55) sets the federal requirements for brakes on commercial motor vehicles. This guide sets out what the rule text states on required service, parking and emergency brake systems, brakes on all wheels and the exceptions, breakaway and emergency braking, air and vacuum reservoirs, gauges and warning signals, brake tubing and hoses, the brake performance table, and the manufacture dates for automatic brake adjusters and antilock brakes. Many paragraphs depend on the vehicle type and its date of manufacture, and several point to Federal Motor Vehicle Safety Standards (FMVSS Nos. 105, 106 and 121) whose content is only named here. This is not driving technique and not your state's knowledge test. Every point links to the regulation and quotes its exact words.
Required brake systems
Each commercial motor vehicle must have brakes adequate to stop and hold the vehicle or combination of vehicles, and must meet the applicable service, parking and emergency brake system requirements of 393.40.
“Each commercial motor vehicle must have brakes adequate to stop and hold the vehicle or combination of motor vehicles. Each commercial motor vehicle must meet the applicable service, parking, and emergency brake system requirements provided in this section.”
Vehicles with hydraulic brakes manufactured on or after September 2, 1983 must, at a minimum, have a service brake system that meets FMVSS No. 105 as in effect on the date of manufacture.
“Motor vehicles equipped with hydraulic brake systems and manufactured on or after September 2, 1983, must, at a minimum, have a service brake system that meets the requirements of FMVSS No. 105 in effect on the date of manufacture.”
Buses, trucks and truck-tractors with air brakes manufactured on or after March 1, 1975, and trailers manufactured on or after January 1, 1975, must, at a minimum, have a service brake system that meets FMVSS No. 121 as in effect on the date of manufacture.
“Buses, trucks and truck-tractors equipped with air brake systems and manufactured on or after March 1, 1975, and trailers manufactured on or after January 1, 1975, must, at a minimum, have a service brake system that meets the requirements of FMVSS No. 121 in effect on the date of manufacture.”
A bus, truck, truck tractor or combination manufactured on or after July 1, 1973 and not covered by 393.40(d)(1) or (d)(2) must have an emergency brake system made of emergency features of the service brake system or a separate emergency system, and it must meet 393.43 and 393.52.
“A bus, truck, truck tractor, or a combination of motor vehicles manufactured on or after July 1, 1973, and not covered under paragraphs (d)(1) or (d)(2) of this section, 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. The emergency brake system must meet the applicable requirements of §§ 393.43 and 393.52.”
Under 393.40(e)(2), the driver must be able to operate the emergency brake control from the normal seating position while restrained by any seat belts the vehicle has. The emergency control may be combined with either the service brake control or the parking brake control, but all three controls may not be combined.
“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. However, all three controls may not be combined.”
Except as provided in 393.49(b) and (c), every motor vehicle manufactured after June 30, 1953 that has power brakes must be arranged so that one application valve, when activated, makes all of the service brakes on the vehicle or combination operate. An additional valve to operate trailer brakes (or as 393.44 requires for buses) is not prohibited.
“Except as provided in paragraphs (b) and (c) of this section, every motor vehicle manufactured after June 30, 1953, which is equipped with power brakes, must have the braking system so arranged that one application valve must when activated cause all of the service brakes on the motor vehicle or combination motor vehicle to operate. This requirement must not be construed to prohibit motor vehicles from being equipped with an additional valve to be used to operate the brakes on a trailer or trailers or as required for buses in § 393.44.”
On every bus with air brakes, the braking system must be built so that if any brake line to any front wheel is broken, the driver can still apply the brakes on the rear wheels. The means to do so may be forward of the driver's seat if the driver can operate it manually while properly restrained by any seat belt provided. Every bus must either meet this requirement or comply with the regulations in effect when it was made.
“On every bus, if equipped with air brakes, the braking system shall be so constructed that in the event any brake line to any of the front wheels is broken, the driver can apply the brakes on the rear wheels despite such breakage. The means used to apply the brakes may be located forward of the driver's seat as long as it can be operated manually by the driver when the driver is properly restrained by any seat belt assembly provided for use. Every bus shall meet this requirement or comply with the regulations in effect at the time of its manufacture.”
Brakes on all wheels and exceptions
Every commercial motor vehicle must have brakes acting on all wheels. This also applies to some vehicles towed in a driveaway-towaway operation: a vehicle towed by tow-bar when another vehicle is full-mounted on it, and any saddlemount configuration with a fullmount.
“Every commercial motor vehicle shall be equipped with brakes acting on all wheels. This requirement also applies to certain motor vehicles being towed in a driveaway-towaway operation, as follows: (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.”
Trucks or truck tractors with three or more axles manufactured before July 25, 1980 do not need brakes on the front wheels, but they must still meet the brake performance rules of 393.52.
“Trucks or truck tractors having three or more axles and manufactured before July 25, 1980, are not required to have brakes on the front wheels. However, these vehicles must meet the requirements of § 393.52.”
Vehicles towed in a driveaway-towaway operation (including the last truck of triple saddle-mount combinations) do not need operative brakes, provided the combination meets 393.52.
“Motor vehicles being towed in a driveaway-towaway operation (including the last truck of triple saddle-mount combinations (see § 393.71(a)(3)) are not required to have operative brakes provided the combination of vehicles meets the requirements of § 393.52.”
A semitrailer or pole trailer (laden or unladen) with a gross weight of 3,000 pounds or less does not need brakes if the axle weight of the towed vehicle does not exceed 40 percent of the sum of the axle weights of the towing vehicle.
“Any semitrailer or pole trailer (laden or unladen) with a gross weight of 1,361 kg (3,000 pounds) or less which is subject to this part is not required to be equipped with brakes if the axle weight of the towed vehicle does not exceed 40 percent of the sum of the axle weights of the towing vehicle.”
A full trailer or four-wheel pole trailer (laden or unladen) with a gross weight of 3,000 pounds or less does not need brakes if the sum of its axle weights does not exceed 40 percent of the sum of the axle weights of the towing vehicle.
“Any full trailer or four-wheel pole trailer (laden or unladen) with a gross weight of 1,361 kg (3,000 pounds) or less which is subject to this part is not required to be equipped with brakes if the sum of the axle weights of the towed vehicle does not exceed 40 percent of the sum of the axle weights of the towing vehicle.”
Portable conveyors manufactured before 2010 and used by the aggregate industry do not need brakes on all wheels, provided all three conditions are met: the combination meets 393.52, the towed vehicle's axle weights do not exceed 40 percent of the towing vehicle's, and speed is limited to 45 mph on two-lane roads and 55 mph on freeways.
“Portable conveyors manufactured prior to 2010 and used by the aggregate industry are not required to be equipped with brakes on all wheels provided: (i) The combination of portable conveyor and towing vehicle meet the performance requirement in 49 CFR 393.52; (ii) The sum of the axle weights of the towed vehicle does not exceed 40 percent of the sum of the axle weights of the towing vehicle; and (iii) The maximum speed of the portable conveyor and towing vehicle is limited to 45 miles per hour (mph) on two lane roads and 55 mph on freeways.”
Except as provided in 393.48(b), (c) and (d), all brakes a vehicle is equipped with must at all times be capable of operating.
“Except as provided in paragraphs (b), (c), and (d) of this section, all brakes with which a motor vehicle is equipped must at all times be capable of operating.”
Manually operated devices to reduce or remove front-wheel braking effort may only be used on buses, trucks and truck tractors manufactured before March 1, 1975, and only when the vehicle is operated under adverse conditions such as wet, snowy or icy roads.
“Manually operated devices to reduce or remove front-wheel braking effort may only be used on buses, trucks, and truck tractors manufactured before March 1, 1975. Such devices must not be used unless the vehicle is being operated under adverse conditions such as wet, snowy, or icy roads.”
An automatic device must not reduce front-wheel braking force by more than 50 percent of the braking force available when the device is disconnected.
“Automatic devices must not reduce the front-wheel braking force by more than 50 percent of the braking force available when the automatic device is disconnected (regardless of whether or not an antilock system failure has occurred on any axle).”
Brakes on lift axles need not work while the lift axle is raised, but they must be capable of being applied whenever the lift axle is lowered and the tires contact the roadway.
“Brakes on lift axles need not be capable of being operated while the lift axle is raised. However, brakes on lift axles must be capable of being applied whenever the lift axle is lowered and the tires contact the roadway.”
Not covered here
The surge brake weight ratios in 393.48(d) and the exception for towed vehicles with disabling damage in 393.48(c) are not covered in detail. We only write what an official source states, so study these parts with your certifying program's materials.
Parking brakes
Hydraulic-braked vehicles manufactured on or after September 2, 1983: each truck and bus (other than a school bus) with a GVWR of 10,000 pounds or less that is subject to Part 393, and school buses with a GVWR over 10,000 pounds, must have a parking brake system as FMVSS No. 571.105 (S5.2) in effect at the time of manufacture requires. The parking brake must be capable of holding the vehicle or combination stationary under any condition of loading in which it is found on a public road (free of ice and snow).
“Hydraulic-braked vehicles manufactured on or after September 2, 1983. Each truck and bus (other than a school bus) with a GVWR of 4,536 kg (10,000 pounds) or less which is subject to this part and school buses with a GVWR greater than 4,536 kg (10,000 pounds) shall be equipped with a parking brake system as required by FMVSS No. 571.105 (S5.2) in effect at the time of manufacture. The parking brake shall be capable of holding the vehicle or combination of vehicles stationary under any condition of loading in which it is found on a public road (free of ice and snow).”
Each air-braked bus, truck and truck tractor manufactured on and after March 1, 1975, and each air-braked trailer (except an agricultural commodity trailer, converter dolly, heavy hauler trailer or pulpwood trailer), must have a parking brake system as FMVSS No. 121 (S5.6) in effect at the time of manufacture requires. The parking brake must be capable of holding the vehicle or combination stationary under any condition of loading in which it is found on a public road (free of ice and snow).
“Each air-braked bus, truck and truck tractor manufactured on and after March 1, 1975, and each air-braked trailer except an agricultural commodity trailer, converter dolly, heavy hauler trailer or pulpwood trailer, shall be equipped with a parking brake system as required by FMVSS No. 121 (S5.6) in effect at the time of manufacture. The parking brake shall be capable of holding the vehicle or combination of vehicles stationary under any condition of loading in which it is found on a public road (free of ice and snow).”
An agricultural commodity trailer, heavy hauler or pulpwood trailer must carry enough chocking blocks to prevent movement when parked.
“An agricultural commodity trailer, heavy hauler or pulpwood trailer shall carry sufficient chocking blocks to prevent movement when parked.”
Paragraph 393.41(c) covers vehicles not subject to FMVSS Nos. 105 and 121 on the date of manufacture. Each singly driven vehicle not subject to the parking brake requirements of FMVSS Nos. 105 or 121 when made, and every combination of vehicles, must have a parking brake system adequate to hold it 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). Under (c)(2), the parking brake must at all times be capable of being applied by either the driver's muscular effort or by spring action; if other energy is used, it must be accumulated isolated from any common source and used only for the parking brake ((c)(2) does not apply to air-applied, mechanically-held parking brakes that meet FMVSS No. 121 (S5.6)). Under (c)(3), the parking brake must be held applied by energy other than fluid pressure, air pressure or electric energy, and must not be releasable unless enough energy is available to reapply it immediately with the required effectiveness.
“Vehicles not subject to FMVSS Nos. 105 and 121 on the date of manufacture. (1) Each singly driven motor vehicle not subject to parking brake requirements of FMVSS Nos. 105 or 121 at the time of manufacturer, and every combination of motor vehicles must be equipped with a parking brake system adequate to hold the vehicle or combination 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). (2) The parking brake system shall, at all times, be capable of being applied by either the driver's muscular effort or by spring action. If other energy is used to apply the parking brake, there must be an accumulation of that energy isolated from any common source and used exclusively for the operation of the parking brake. Exception: This paragraph shall not be applicable to air-applied, mechanically-held parking brake systems which meet the parking brake requirements of FMVSS No. 121 (S5.6). (3) The parking brake system shall be held in the applied position by energy other than fluid pressure, air pressure, or electric energy. The parking brake system shall not be capable of being released unless adequate energy is available to immediately reapply the parking brake with the required effectiveness.”
Breakaway and emergency braking
Every vehicle used to tow a trailer equipped with brakes must have a means so that, if the trailer breaks away, the service brakes on the towing vehicle can still stop the towing vehicle. On air-braked towing units, the tractor protection valve or similar device must operate automatically when the towing vehicle's air pressure is between 20 psi and 45 psi.
“Every motor vehicle, if used to tow a trailer equipped with brakes, shall be equipped with a means for providing that in the case of a breakaway of the trailer, the service brakes on the towing vehicle will be capable of stopping the towing vehicle. For air braked towing units, the tractor protection valve or similar device shall operate automatically when the air pressure on the towing vehicle is between 138 kPa and 310 kPa (20 psi and 45 psi).”
Every air-braked truck or truck tractor towing air-braked vehicles must have two means of activating the trailer's emergency brakes: one that operates automatically when the towing vehicle's air supply drops to a fixed pressure not lower than 20 psi nor higher than 45 psi, and a manually controlled device readily operable by a person seated in the driving seat.
“Every truck or truck tractor equipped with air brakes, when used for towing other vehicles equipped with air brakes, shall be equipped with two means of activating the emergency features of the trailer brakes. One of these means shall operate automatically in the event of reduction of the towing vehicle air supply to a fixed pressure which shall not be lower than 20 pounds per square inch nor higher than 45 pounds per square inch. The other means shall be a manually controlled device readily operable by a person seated in the driving seat.”
The manual device's emergency position or method of operation must be clearly indicated, and it may never be arranged so it can prevent the automatic means from operating. The two means may be separate, but they do not have to be.
“Its emergency position or method of operation shall be clearly indicated. In no instance may the manual means be so arranged as to permit its use to prevent operation of the automatic means. The automatic and manual means required by this section may be, but are not required to be, separate.”
Every trailer required to have brakes must have brakes that apply automatically and immediately upon breakaway from the towing vehicle. Except on trailers with three or more axles, all of the trailer's required brakes must apply on breakaway. The brakes must stay applied for at least 15 minutes.
“Every trailer required to be equipped with brakes shall have brakes which apply automatically and immediately upon breakaway from the towing vehicle. With the exception of trailers having three or more axles, all brakes with which the trailer is required to be equipped must be applied upon breakaway from the towing vehicle. The brakes must remain in the applied position for at least 15 minutes.”
Air brake systems on towed vehicles must use no-bleed-back relay emergency valves or equivalent devices, so the trailer's supply reservoir is protected against air flowing back to the towing vehicle when the towing vehicle's air pressure drops.
“Air brake systems installed on towed vehicles shall be so designed, by the use of “no-bleed-back” relay emergency valves or equivalent devices, that the supply reservoir used to provide air for brakes shall be safeguarded against backflow of air to the towing vehicle upon reduction of the towing vehicle air pressure.”
The two-means, vacuum second-control and trailer breakaway rules (393.43(b), (c) and (d)) do not apply to vehicles being transported in driveaway-towaway operations.
“The requirements of paragraphs (b), (c) and (d) of this section shall not be applicable to commercial motor vehicles being transported in driveaway-towaway operations.”
Reservoirs, gauges and warning signals
For air-braked vehicles not subject to FMVSS No. 121 when made, and all vacuum-braked vehicles, the reserve capacity or reservoir must let the driver make a full service brake application with the engine stopped without dropping the air pressure or vacuum below 70 percent of the gauge reading just before the application. A full service application means pressing the brake pedal or treadle valve to the limit of its travel.
“Reservoir capacity for air-braked vehicles not subject to FMVSS No. 121 on the date of manufacture and all vacuum braked vehicles. Each motor vehicle using air or vacuum braking must have either reserve capacity, or a reservoir, that would enable the driver to make a full service brake application with the engine stopped without depleting the air pressure or vacuum below 70 percent of that indicated by the air or vacuum gauge immediately before the brake application is made. For the purposes of this paragraph, a full service brake application means depressing the brake pedal or treadle valve to the limit of its travel.”
Each service reservoir system must be protected against loss of air pressure or vacuum from a failure or leak between the reservoir and the source, by check valves or equivalent devices that can be checked without disconnecting any air or vacuum line or fitting.
“Each service reservoir system on a motor vehicle shall be protected against a loss of air pressure or vacuum due to a failure or leakage in the system between the service reservoir and the source of air pressure or vacuum, by check valves or equivalent devices whose proper functioning can be checked without disconnecting any air or vacuum line, or fitting.”
On air-braked vehicles, each reservoir must have a condensate drain valve that can be manually operated. Automatic drain valves may be used if one of two options is met: they can be operated manually, or a manual means of draining the reservoirs is kept.
“Drain valves for air braked vehicles. Each reservoir must have a condensate drain valve that can be manually operated. Automatic condensate drain valves may be used provided (1) they may be operated manually, or (2) a manual means of draining the reservoirs is retained.”
Every bus, truck and truck tractor, except as provided in 393.51(f), must have a signal that warns the driver when a failure occurs in the service brake system.
“Every bus, truck and truck tractor, except as provided in paragraph (f), must be equipped with a signal that provides a warning to the driver when a failure occurs in the vehicle's service brake system.”
49 CFR 393.51, Warning signals, air pressure and vacuum gauges
A vehicle with air brakes, or one towing a vehicle with air brakes, must have a pressure gauge and a warning signal, whatever its date of manufacture.
“A commercial motor vehicle (regardless of the date of manufacture) equipped with service brakes activated by compressed air (air brakes) or a commercial motor vehicle towing a vehicle with service brakes activated by compressed air (air brakes) must be equipped with a pressure gauge and a warning signal.”
49 CFR 393.51, Warning signals, air pressure and vacuum gauges
Power units to which FMVSS No. 121 did not apply when made must have a pressure gauge visible from the normal driving position, and a warning signal (audible or visible) that gives a continuous warning whenever service reservoir air pressure is at 55 psi and below, or one-half of the compressor governor cutout pressure, whichever is less.
“Power units to which FMVSS No. 571.121 was not applicable on the date of manufacture of the vehicle must be equipped with— (1) A pressure gauge, visible to a person seated in the normal driving position, which indicates the air pressure (in kilopascals (kPa) or pounds per square inch (psi)) available for braking; and (2) A warning signal that is audible or visible to a person in the normal driving position and provides a continuous warning to the driver whenever the air pressure in the service reservoir system is at 379 kPa (55 psi) and below, or one-half of the compressor governor cutout pressure, whichever is less.”
49 CFR 393.51, Warning signals, air pressure and vacuum gauges
A vehicle with vacuum-activated service brakes, or one towing such a vehicle, must have (whatever its date of manufacture) a vacuum gauge visible from the normal driving position and a warning signal that gives a continuous warning whenever the vacuum in the supply reservoir is less than 8 inches of mercury.
“A commercial motor vehicle (regardless of the date it was manufactured) having service brakes activated by vacuum or a vehicle towing a vehicle having service brakes activated by vacuum must be equipped with— (1) A vacuum gauge, visible to a person seated in the normal driving position, which indicates the vacuum (in millimeters or inches of mercury) available for braking; and (2) A warning signal that is audible or visible to a person in the normal driving position and provides a continuous warning to the driver whenever the vacuum in the vehicle's supply reservoir is less than 203 mm (8 inches) of mercury.”
49 CFR 393.51, Warning signals, air pressure and vacuum gauges
The air, vacuum and air- or vacuum-assisted hydraulic gauge and signal rules (393.51(c), (d) and (e)) do not apply to property-carrying vehicles with fewer than three axles that were manufactured before July 1, 1973 and have a manufacturer's GVWR under 10,001 pounds.
“The rules in paragraphs (c), (d) and (e) of this section do not apply to property carrying commercial motor vehicles which have less than three axles and (1) were manufactured before July 1, 1973, and (2) have a manufacturer's gross vehicle weight rating less than 4,536 kg (10,001 pounds).”
49 CFR 393.51, Warning signals, air pressure and vacuum gauges
Not covered here
The hydraulic brake warning lamp rules of 393.51(b) and (e) and the content of FMVSS Nos. 105 and 121 are not summarized here. We only write what an official source states, so study these parts with your certifying program's materials.
Brake tubing, hoses and components
All brake tubing and hoses, brake hose assemblies and end fittings must meet FMVSS No. 106.
“All brake tubing and hoses, brake hose assemblies, and brake hose end fittings must meet the applicable requirements of FMVSS No. 106 (49 CFR 571.106).”
49 CFR 393.45, Brake tubing and hoses; hose assemblies and end fittings
Brake tubing and hose must be long and flexible enough to handle all normal motions of the parts it is attached to without damage, secured against chafing, kinking or other mechanical damage, and installed so it cannot contact the exhaust system or any other source of high temperatures.
“Brake tubing and hose must— (1) Be long and flexible enough to accommodate without damage all normal motions of the parts to which it is attached; (2) Be secured against chafing, kinking, or other mechanical damage; and (3) Be installed in a manner that prevents it from contacting the vehicle's exhaust system or any other source of high temperatures.”
49 CFR 393.45, Brake tubing and hoses; hose assemblies and end fittings
All connections for air, vacuum or hydraulic braking systems must be installed so the attachment is free of leaks, constrictions or other conditions that would hurt brake performance.
“All connections for air, vacuum, or hydraulic braking systems shall be installed so as to ensure an attachment free of leaks, constrictions or other conditions which would adversely affect the performance of the brake system.”
49 CFR 393.45, Brake tubing and hoses; hose assemblies and end fittings
Brake components must be built, installed and maintained to prevent excessive fading and grabbing. The service and spring brake chambers on each end of an axle must be the same size, and the slack adjusters on each end of an axle must have the same effective length.
“Brake components must be constructed, installed and maintained to prevent excessive fading and grabbing. The means of attachment and physical characteristics must provide for safe and reliable stopping of the commercial motor vehicle. (b) Brake chambers. The service brake chambers and spring brake chambers on each end of an axle must be the same size. (c) Slack adjusters. The effective length of the slack adjuster on each end of an axle must be the same.”
49 CFR 393.47, Brake actuators, slack adjusters, linings/pads and drums/rotors
On the steering axle of a truck, truck-tractor or bus with air drum brakes, lining/pad thickness must not be less than 3/16 inch at the shoe center for a shoe with a continuous strip of lining, less than 1/4 inch at the shoe center for a shoe with two pads, or worn to the wear indicator if the lining is so marked. Steering axle lining/pad thickness must not be less than 1/8 inch for air disc brakes, or 1/16 inch or less for hydraulic disc, drum and electric brakes.
“Steering axle brakes. The brake lining/pad thickness on the steering axle of a truck, truck-tractor or bus shall not be less than 4.8 mm (3⁄16 inch) at the shoe center for a shoe with a continuous strip of lining; less than 6.4 mm (1⁄4 inch) at the shoe center for a shoe with two pads; or worn to the wear indicator if the lining is so marked, for air drum brakes. The steering axle brake lining/pad thickness shall not be less than 3.2 mm (1⁄8 inch) for air disc brakes, or 1.6 mm (1⁄16 inch) or less for hydraulic disc, drum and electric brakes.”
49 CFR 393.47, Brake actuators, slack adjusters, linings/pads and drums/rotors
On non-steering axles, an air-braked vehicle must not be operated with lining or pad thickness under 1/4 inch or worn to the wear indicator if the lining is marked (measured at the shoe center for drum brakes), or under 1/8 inch for disc brakes. Hydraulic or electric braked vehicles must not be operated with lining or pad thickness under 1/16 inch (measured at the shoe center) for disc or drum brakes.
“Non-steering axle brakes. An air braked commercial motor vehicle shall not be operated with brake lining/pad thickness less than 6.4 mm (1⁄4 inch) or to the wear indicator if the lining is so marked (measured at the shoe center for drum brakes); or less than 3.2 mm (1⁄8 inch) for disc brakes. Hydraulic or electric braked commercial motor vehicles shall not be operated with a lining/pad thickness less than 1.6 mm (1⁄16 inch) (measured at the shoe center) for disc or drum brakes.”
49 CFR 393.47, Brake actuators, slack adjusters, linings/pads and drums/rotors
Not covered here
The pushrod stroke readjustment-limit tables in 393.47(e), the wedge brake and drum/rotor limits in 393.47(f) and (g), and the coiled nonmetallic tubing details in 393.45(c) are not reproduced here; read them in the regulation. We only write what an official source states, so study these parts with your certifying program's materials.
Brake performance
On applying its service brakes, a vehicle or combination must, under any condition of loading found on a public highway, be able to develop the braking force, decelerate from 20 mph at the rate, and stop from 20 mph within the distance (measured from when the brake pedal or control starts to move) set in the 393.52(d) table. Vehicles or combinations with a GVWR or GVW over 10,000 pounds have the alternative in the next point.
“Upon application of its service brakes, a motor vehicle or combination of motor vehicles must under any condition of loading in which it is found on a public highway, be capable of— (1) Developing a braking force at least equal to the percentage of its gross weight specified in the table in paragraph (d) of this section; (2) Decelerating to a stop from 20 miles per hour at not less than the rate specified in the table in paragraph (d) of this section; and (3) Stopping from 20 miles per hour in a distance, measured from the point at which movement of the service brake pedal or control begins, that is not greater than the distance specified in the table in paragraph (d) of this section; or, for motor vehicles or motor vehicle combinations that have a GVWR or GVW greater than 4,536 kg (10,000 pounds),”
As an alternative for vehicles or combinations with a GVWR or GVW over 10,000 pounds, they need only develop the table's braking force and stopping distance if braking force is measured by a performance-based brake tester that meets the functional specifications for such testers.
“or, for motor vehicles or motor vehicle combinations that have a GVWR or GVW greater than 4,536 kg (10,000 pounds), (4) Developing only the braking force specified in paragraph (a)(1) of this section and the stopping distance specified in paragraph (a)(3) of this section, if braking force is measured by a performance-based brake tester which meets the requirements of functional specifications for performance-based brake testers for commercial motor vehicles”
With only its emergency brake system applied, a vehicle or combination must, under any condition of loading found on a public highway, be able to stop from 20 mph within the table's distance, measured from when the emergency brake control starts to move.
“Upon application of its emergency brake system and with no other brake system applied, a motor vehicle or combination of motor vehicles must, under any condition of loading in which it is found on a public highway, be capable of stopping from 20 miles per hour in a distance, measured from the point at which movement of the emergency brake control begins, that is not greater than the distance specified in the table in paragraph (d) of this section.”
Conformity to the stopping-distance requirements of 393.52(a) and (b) is determined with the test made on a hard surface that is substantially level, dry, smooth and free of loose material, with the vehicle starting in the center of a 12-foot-wide lane and not leaving that lane during the test.
“Conformity to the stopping-distance requirements of paragraphs (a) and (b) of this section shall be determined under the following conditions: (1) Any test must be made with the vehicle on a hard surface that is substantially level, dry, smooth, and free of loose material. (2) The vehicle must be in the center of a 12-foot-wide lane when the test begins and must not deviate from that lane during the test.”
Table row A(3), all other passenger-carrying vehicles: braking force 43.5 percent of gross weight, deceleration 14 feet per second per second, service brake application and braking distance 35 feet from 20 mph, emergency brake distance 85 feet.
“(3) All other passenger-carrying vehicles 43.5 14 35 85”
Table row B(1), single unit property-carrying vehicles with a manufacturer's GVWR of 10,000 pounds or less: braking force 52.8 percent, deceleration 17 feet per second per second, service distance 25 feet, emergency distance 66 feet.
“(1) Single unit vehicles having a manufacturer's GVWR of 10,000 pounds or less 52.8 17 25 66”
Table row B(3), all other property-carrying vehicles and combinations of property-carrying vehicles: braking force 43.5 percent, deceleration 14 feet per second per second, service brake application and braking distance 40 feet from 20 mph, emergency brake distance 90 feet.
“(3) All other property-carrying vehicles and combinations of property-carrying vehicles 43.5 14 40 90”
Not covered here
Rows A(1), A(2) and B(2) of the 393.52(d) table and the notes under the table are not summarized here; read the full vehicle descriptions in the regulation. We only write what an official source states, so study these parts with your certifying program's materials.
Automatic brake adjusters and ABS
Each commercial motor vehicle with hydraulic brakes manufactured on or after October 20, 1993 must meet the automatic brake adjustment requirements of FMVSS No. 105 that applied when it was made.
“Each commercial motor vehicle manufactured on or after October 20, 1993, and equipped with a hydraulic brake system, shall meet the automatic brake adjustment system requirements of Federal Motor Vehicle Safety Standard No. 105 (49 CFR 571.105, S5.1) applicable to the vehicle at the time it was manufactured.”
49 CFR 393.53, Automatic brake adjusters and brake adjustment indicators
Each commercial motor vehicle with air brakes manufactured on or after October 20, 1994 must meet the automatic brake adjustment requirements of FMVSS No. 121 that applied when it was made.
“Each commercial motor vehicle manufactured on or after October 20, 1994, and equipped with an air brake system must meet the automatic brake adjustment system requirements of Federal Motor Vehicle Safety Standard No. 121 (49 CFR 571.121, S5.1.8 or S5.2.2) applicable to the vehicle at the time it was manufactured.”
49 CFR 393.53, Automatic brake adjusters and brake adjustment indicators
On air-braked vehicles manufactured on or after October 20, 1994 that have an external automatic adjustment mechanism and an exposed pushrod, service brake under-adjustment must be shown by a brake adjustment indicator meeting FMVSS No. 121.
“On each commercial motor vehicle manufactured on or after October 20, 1994, and equipped with an air brake system which contains an external automatic adjustment mechanism and an exposed pushrod, the condition of service brake under-adjustment must be displayed by a brake adjustment indicator conforming to the requirements of Federal Motor Vehicle Safety Standard No. 121 (49 CFR 571.121, S5.1.8 or S5.2.2) applicable to the vehicle at the time it was manufactured.”
49 CFR 393.53, Automatic brake adjusters and brake adjustment indicators
Each truck and bus with hydraulic brakes manufactured on or after March 1, 1999 (except those in driveaway-towaway operations) must have an antilock brake system meeting FMVSS No. 105.
“Each truck and bus manufactured on or after March 1, 1999 (except trucks and buses engaged in driveaway-towaway operations), and equipped with a hydraulic brake system, shall be equipped with an antilock brake system that meets the requirements of Federal Motor Vehicle Safety Standard (FMVSS) No. 105 (49 CFR 571.105, S5.5).”
Each truck tractor manufactured on or after March 1, 1997 (except those in driveaway-towaway operations) must have an antilock brake system meeting FMVSS No. 121.
“Each truck tractor manufactured on or after March 1, 1997 (except truck tractors engaged in driveaway-towaway operations), shall be equipped with an antilock brake system that meets the requirements of FMVSS No. 121 (49 CFR 571.121, S5.1.6.1(b)).”
Each air-braked commercial motor vehicle other than a truck tractor manufactured on or after March 1, 1998 (except those in driveaway-towaway operations) must have an antilock brake system meeting FMVSS No. 121.
“Each air braked commercial motor vehicle other than a truck tractor, manufactured on or after March 1, 1998 (except commercial motor vehicles engaged in driveaway-towaway operations), shall be equipped with an antilock brake system that meets the requirements of FMVSS No. 121 (49 CFR 571.121, S5.1.6.1(a) for trucks and buses, S5.2.3 for semitrailers, converter dollies and full trailers).”
Each trailer (including a trailer converter dolly) manufactured on or after March 1, 1998 and subject to 393.55(c)(2) must have an exterior ABS malfunction indicator lamp meeting FMVSS No. 121.
“Exterior ABS malfunction indicator lamps for trailers. Each trailer (including a trailer converter dolly) manufactured on or after March 1, 1998, and subject to the requirements of paragraph (c)(2) of this section, shall be equipped with an ABS malfunction indicator lamp which meets the requirements of FMVSS No. 121 (49 CFR 571.121, S5.2.3.3).”
Not covered here
The ABS malfunction circuit and signal rules of 393.55(b) and (d) are not covered in detail. We only write what an official source states, so study these parts with your certifying program's materials.
Sources
- eCFR: 49 CFR 393.40, Required brake systems (as of Nov 3, 2026)Checked November 6, 2026
- eCFR: 49 CFR 393.49, Control valves for brakes (as of Nov 3, 2026)Checked November 6, 2026
- eCFR: 49 CFR 393.44, Front brake lines, protection (as of Nov 3, 2026)Checked November 6, 2026
- eCFR: 49 CFR 393.42, Brakes required on all wheels (as of Nov 3, 2026)Checked November 6, 2026
- eCFR: 49 CFR 393.48, Brakes to be operative (as of Nov 3, 2026)Checked November 6, 2026
- eCFR: 49 CFR 393.41, Parking brake system (as of Nov 3, 2026)Checked November 6, 2026
- eCFR: 49 CFR 393.43, Breakaway and emergency braking (as of Nov 3, 2026)Checked November 6, 2026
- eCFR: 49 CFR 393.50, Reservoirs required (as of Nov 3, 2026)Checked November 6, 2026
- eCFR: 49 CFR 393.51, Warning signals, air pressure and vacuum gauges (as of Nov 3, 2026)Checked November 6, 2026
- eCFR: 49 CFR 393.45, Brake tubing and hoses; hose assemblies and end fittings (as of Nov 3, 2026)Checked November 6, 2026
- eCFR: 49 CFR 393.47, Brake actuators, slack adjusters, linings/pads and drums/rotors (as of Nov 3, 2026)Checked November 6, 2026
- eCFR: 49 CFR 393.52, Brake performance (as of Nov 3, 2026)Checked November 6, 2026
- eCFR: 49 CFR 393.53, Automatic brake adjusters and brake adjustment indicators (as of Nov 3, 2026)Checked November 6, 2026
- eCFR: 49 CFR 393.55, Antilock brake systems (as of Nov 3, 2026)Checked November 6, 2026
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