Air Brake Related Regulations and Vehicle Safety
Regulation questions test the standards a roadside inspector applies and the driver duties that go with them.
What the rules require
Federal rules adopted in Maine (49 CFR 393 Subpart C and Part 396) require every CMV to have service, parking, and emergency brake systems in good working order. The parking brake must hold the vehicle on a 20% grade. Service brakes must stop from 20 mph within 30 feet on a dry, level, hard surface. Every CMV needs a periodic inspection at least every 12 months, and brake work must be done or supervised by a qualified brake inspector (396.25).
Out-of-service criteria
CVSA roadside inspections place a vehicle out of service for:
- 20% or more of the brakes defective on the vehicle or combination
- Push rod travel beyond the adjustment limit at any chamber
- Air loss of 3 psi/min or more on a single vehicle, or 4 psi/min or more on a combination
- A low-pressure warning device that fails to operate before 60 psi
- Spring brakes that do not apply in the 20–45 psi range
- Any missing, cracked, or contaminated brake component
Driver duties
Pre-trip and post-trip inspections are required, along with a written DVIR when a defect is found and an immediate report to the carrier. A driver may not dispatch a vehicle with a known brake defect. Brake lines, chambers, and slack adjusters must be secure, and linings must not be oil-soaked.
Worked example
A 10-brake combination arrives with both steer brakes and one drive brake out of adjustment. That is 3 defective brakes out of 10 - 30% - which exceeds the 20% threshold, so the vehicle is out of service and must be repaired and re-inspected before it moves.
Traps
Do not confuse the 20% brake rule with a 25% or 50% rule, and do not treat a DVIR as paperwork you can sign without walking the vehicle. A "no defects" report still requires the inspection.