§393.43 requires every towed vehicle (other than a pole trailer or coupled to a converter dolly) to be equipped with brakes acting on all wheels (per §393.42), and requires a means by which the brakes on the towed vehicle will be automatically applied if the towed vehicle breaks away from the towing vehicle. For air-braked combinations, this is the emergency-brake application that occurs on loss of air pressure; for hydraulic-braked trailers, this is the breakaway-cable-actuated emergency brake.
OOS
A non-functional breakaway brake system on a towed vehicle is an OOS condition. The combination cannot continue until the protection is restored — emergency brake application must occur on disconnection of the air or signal line.
Severity weight
Severity weight 6 in the Vehicle Maintenance BASIC. The weight reflects the severity of the failure mode: a runaway trailer in traffic.
How to prevent it
- Include a "yank test" in pre-trip — pull the emergency air line and confirm the trailer brakes apply.
- For hydraulic trailers, the breakaway battery is the failure mode — check it on every pre-trip.
- After any trailer disconnect/reconnect, re-verify breakaway protection before moving.
How Roadworthy HQ helps
§393.43 findings logged in Roadworthy HQ link to the trailer and block dispatch under §396.11(c) until §396.9(d)(3) repair certification is on file. The DVIR pre-trip checklist surfaces the breakaway test as a required step.