U.S. Federal regulations specify the adequacy and application of brake related systems and parts. Do not rely upon the data here to be current. Always check for the latest revisions, exceptions and for additional applicable regulations.

US Air Brake Regulations
What Is "121" Breakaway & Emergency Brakes (393.43) Required Brake Systems (393.40)
Complete 121
Brakes Required On All Wheels (393.42) Brake Tube & Hose, Adequacy (393.45)
Air Suspensions
(393.207 Section F)
Parking Brakes
Anti-Lock Brakes

Lighting Regulation Downloads
Lamps And Reflective Devices
Retroreflective Sheeting & Reflex Reflectors
Lamps & Reflectors On Driveaway/Towaway Operations (393.17)
Combining Lighting Devices & Reflectors.
Requirements For Lamps Other Than Head Lamps
Requirements For Reflex Reflectors.
Wiring Specifications

Canada Air Brake Regulations
Technical Standard Document # 121 Ontario SPIF Ontario FAQ