FMVSS 121

Air Brake Systems

This Standard applies to trucks, buses and trailers equipped with air brake systems. It establishes performance and equipment requirements for braking systems on vehicles equipped with air brake systems, including separately controlled parking brakes of prescribed holding capability, automatic brake adjusters and adjustment indicators, and emergency brakes which activate automatically in the event of loss of air pressure. Anti-lock brakes are required on new tractors with a GVWR over 10,000 lbs. (4,536 kg) beginning March 1, 1997. Anti-lock brakes are required on new trailers, single-unit trucks and buses with a GVWR greater than 10,000 lbs. (4,536 kg) with air brakes beginning March 1, 1998. Automatic activation of the air compressor is required whenever the pressure in the air brake system
drops below 100 psi starting March 1, 1997.

Note: The Standard does not apply to: (1) Any trailer that has a width of more than 102.36" with extendable equipment in the fully retracted position and is equipped with two short-track axles in a line across the width of the trailer; (2) Any vehicle equipped with an axle that has a GAWR of 29,000 lbs.; (3) Any truck or bus that has a speed attainable in two miles of not more than 33 mph; (4) Any truck that has a speed attainable in two miles of not more than 45 mph, an unloaded vehicle weight that is not less than 95% of its GVWR and no capacity to carry occupants other than the driver and operating crew; (5) Any trailer that has a GVWR more than 120,000 lbs. and whose body conforms to that of a heavy hauler trailer; (6) Any trailer that has an unloaded vehicle weight that is not less than 95% of its GVWR; (7) Any load divider dolly.

Applicability:

Trucks & MPVs 10,000 GVWR or Less No 
Trucks More than 10,000 GVWR Yes
Buses 10,000 GVWR or Less No 
Buses More than 10,000 GVWR Yes
Trailers Yes

Read the Complete Standard