When I pull apart a trailer after a roadside write-up for brake line damage, the failed spot is almost never where the driver's eyes went during the walk-around. It is usually a few inches from a frame crossmember, a spring hanger, or a tire sidewall, rubbing a little more with every mile. The code is 393.45B2UV, "Brake Hose or Tubing Chafing and/or Kinking Under Vehicle." It falls under the Vehicle Maintenance BASIC, in the Brakes, All Others violation group, with a severity weight of 4.
You can see the full record for this code on its violation page. I want to walk through the mechanics first, because the code makes more sense once you know what the hose is doing.
What the regulation is asking for
Section 393.45 governs brake tubing and hose adequacy. In substance, brake lines must be installed and protected so they are not chafed, kinked, or otherwise damaged, and they must be in serviceable condition. The rule does not say "no hose may ever touch anything." It says the hose must be routed and secured so that contact and bending do not wear it down. You can read the text directly on eCFR's 49 CFR 393.45.
The inspector's question is simple: is the line protected, and is it in condition to hold pressure?
Why chafing matters on air brakes
On an air brake system, compressed air travels through the hose to the brake chamber at each wheel. A kink restricts that flow. A restricted line can slow application and release at the chamber, and the driver may not notice until the system is under load. That is a brake performance problem, not a cosmetic one.
Chafing is slower but more predictable. A line that rubs against steel or a tire loses its outer jacket first. Then the reinforcement braid is exposed. Once the braid is cut through, the line can leak, and in a bad case it can burst. Heat, cold, and age all make rubber less forgiving, so a hose that rubs marginally in mild weather can fail faster in a cold snap.
Chafe points usually come from three things:
- Routing that changed. Someone reroutes a line during a repair and the new path crosses a moving component.
- Loose or missing clips. A clip that breaks off lets the hose sag into the road path or swing into a frame edge.
- Tie-straps pulled too tight. A tight strap concentrates bending at one point, and the hose cracks there.
A useful habit: after any tire work, suspension repair, or axle service, go back and check the lines in that area before you roll.
What the numbers say
In the current dataset, this code has 190,913 recorded violations and 18,732 out-of-service results, which works out to the 9.8 percent OOS rate shown on the violation page. The inspection count tied to the code is 145,250. Those are the figures in the grounding data, and they tell you two things.
First, the OOS indicator on this code is off. That means the code is not automatically out-of-service on its own. The inspector judges the condition. A hose with light surface wear will often be written as a violation without an OOS placement. A hose with the braid showing, a leak, or a kink that visibly restricts the line is a different call. About one in ten recorded violations of this code ended in an out-of-service placement.
Second, the code carries real weight in your Safety Measurement System profile. It is used in SMS with a severity weight of 4. The calculator applies a time-weight tier to each violation based on age: 3 for 0 to 6 months, 2 for 6 to 12 months, 1 for 12 to 24 months, and 0 beyond 24 months. Severity contributions are capped at 30 per inspection per BASIC, so one bad inspection with many brake write-ups cannot grow without limit. If you are unfamiliar with BASIC terms or how OOS is defined, the glossary is the place to start.
What I check on a walk-around
When I inspect lines on a trailer, I am looking for specific things rather than a general sense that something looks off:
- Shiny or flattened spots on the outer jacket where the hose has rubbed.
- Sharp bends right at a fitting, which usually means the hose is too short for its path or has been pulled tight.
- Sagging low enough to touch the road, a tire, or a mud flap.
- Tie-straps biting into the hose or missing entirely.
- Clips broken or sitting loose on their mounts.
Use a flashlight and look at the whole length, including the parts behind the axles that are hard to see from the side. Do not pull hard on a line to test it. Gentle hand pressure on a sound hose is fine; yanking on a worn one can finish the job.
When you fix a chafe point, re-route or re-clip the line so the contact stops. Add a protective sleeve where needed. If the outer jacket is cut or the braid is showing, replace that section. Wrapping damaged hose in tape is not a repair and it will be written up.
Key takeaways
- 393.45B2UV covers chafing and kinking of brake hose and tubing under the vehicle, in the Vehicle Maintenance BASIC, with a severity weight of 4.
- The code is not automatically out-of-service. The OOS call depends on the condition the inspector finds, and about 9.8 percent of recorded violations in this dataset ended in OOS.
- The most common cause is routing and clip failure, not bad hose out of the box.
- Check the lines after any tire, suspension, or axle work, because that is when routing changes.