Federal Motor Carrier Safety Administration, DOT § 393.42

subject to the parking brake require- leased unless adequate energy is avail- ments of FMVSS No. 571.105 (S5.2) must able to immediately reapply the park- be equipped with a parking brake sys- ing brake with the required effective- tem that meets the requirements of ness. paragraph (c) of this section. (b) Air-braked power units manufac- [70 FR 48048, Aug. 15, 2005] tured on or after March 1, 1975, and air- § 393.42 Brakes required on all wheels. braked trailers manufactured on or after January 1, 1975. Each air-braked bus, (a) Every commercial motor vehicle truck and truck tractor manufactured shall be equipped with brakes acting on on and after March 1, 1975, and each all wheels. air-braked trailer except an agricul- (b) Exception. (1) Trucks or truck tural commodity trailer, converter tractors having three or more axles dolly, heavy hauler trailer or pulpwood and manufactured before July 25, 1980, trailer, shall be equipped with a park- are not required to have brakes on the ing brake system as required by front wheels. However, these vehicles FMVSS No. 121 (S5.6) in effect at the must meet the requirements of § 393.52. time of manufacture. The parking (2) Motor vehicles being towed in a brake shall be capable of holding the driveaway-towaway operation are not vehicle or combination of vehicles sta- required to have operative brakes pro- tionary under any condition of loading vided the combination of vehicles in which it is found on a public road meets the requirements of § 393.52. This (free of ice and snow). An agricultural exception is not applicable to: commodity trailer, heavy hauler or (i) Any motor vehicle towed by pulpwood trailer shall carry sufficient means of a tow-bar when another chocking blocks to prevent movement motor vehicle is full-mounted on the when parked. towed vehicle; and (c) Vehicles not subject to FMVSS Nos. (ii) Any combination of motor vehi- 105 and 121 on the date of manufacture. cles utilizing three or more saddle- (1) Each singly driven motor vehicle mounts. not subject to parking brake require- (3) Any semitrailer or pole trailer ments of FMVSS Nos. 105 or 121 at the (laden or unladen) with a gross weight time of manufacturer, and every com- of 1,361 kg (3,000 pounds) or less which bination of motor vehicles must be is subject to this part is not required to equipped with a parking brake system be equipped with brakes if the axle adequate to hold the vehicle or com- weight of the towed vehicle does not bination on any grade on which it is exceed 40 percent of the sum of the axle operated, under any condition of load- weights of the towing vehicle. ing in which it is found on a public (4) Any full trailer or four-wheel pole road (free of ice and snow). trailer (laden or unladen) with a gross (2) The parking brake system shall, weight of 1,361 kg (3,000 pounds) or less at all times, be capable of being applied which is subject to this part is not re- by either the driver’s muscular effort quired to be equipped with brakes if or by spring action. If other energy is the sum of the axle weights of the used to apply the parking brake, there towed vehicle does not exceed 40 per- must be an accumulation of that en- cent of the sum of the axle weights of ergy isolated from any common source the towing vehicle. and used exclusively for the operation (5) Brakes are not required on the of the parking brake. steering axle of a three-axle dolly Exception: This paragraph shall not be which is steered by a co-driver. applicable to air-applied, mechani- (6) Loaded housemoving dollies, spe- cally-held parking brake systems cialized trailers and dollies used to which meet the parking brake require- transport industrial furnaces, reactors, ments of FMVSS No. 121 (S5.6). and similar motor vehicles are not re- (3) The parking brake system shall be quired to be equipped with brakes, pro- held in the applied position by energy vided the speed at which the combina- other than fluid pressure, air pressure, tion of vehicles will be operated does or electric energy. The parking brake not exceed 32 km/hour (20 mph) and system shall not be capable of being re- brakes on the combination of vehicles

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are capable of stopping the combina- operated or 32 km/hour (20 mph), tion within 12.2 meters (40 feet) from whichever is less. the speed at which the vehicle is being

[52 FR 2803, Jan. 27, 1987, as amended at 53 FR 49398, Dec. 7, 1988; 54 FR 48617, Nov. 24, 1989; 59 FR 25574, May 17, 1994; 61 FR 1843, Jan. 24, 1996; 70 FR 48049, Aug. 15, 2005]

§ 393.43 Breakaway and emergency ing units, the tractor protection valve braking. or similar device shall operate auto- (a) Towing vehicle protection system. matically when the air pressure on the Every motor vehicle, if used to tow a towing vehicle is between 138 kPa and trailer equipped with brakes, shall be 310 kPa (20 psi and 45 psi). equipped with a means for providing (b) Emergency brake requirements, air that in the case of a breakaway of the brakes. Every truck or truck tractor trailer, the service brakes on the tow- equipped with air brakes, when used for ing vehicle will be capable of stopping towing other vehicles equipped with air the towing vehicle. For air braked tow- brakes, shall be equipped with two

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