2.18.1 – How Antilock Braking Systems Work Sensors detect potential wheel lockup. An electronic control unit (ECU) will then decrease brake pressure to avoid wheel lockup. Brake pressure is adjusted to provide the maximum braking without danger of lockup. ABS works far faster than the driver can respond to potential wheel lockup. At all other times the brake system will operate normally. 2.18.2 – Vehicles Required to Have Antilock Braking Systems The Department of Transportation requires that ABS be on Truck tractors with air brakes built on or after March 1, 1997. Other air brake vehicles, (trucks, buses, trailers, and converter dollies) built on or after March 1, 1998. Hydraulically braked trucks and buses with a gross vehicle weight rating of 10,000 lbs or more built on or after March 1, 1999. Many commercial vehicles built before these dates have been voluntarily equipped with ABS.