What 393.47(b) means in plain language
This regulation covers the components that physically create stopping force on your truck — specifically the actuators, chambers, and related hardware that convert air pressure into brake application at each wheel end. When an inspector determines that any of these components are defective or failing to perform their intended function, you can be cited under 393.47(b).
In practical terms, this means a cracked, corroded, or loose brake chamber; a diaphragm that's torn or leaking air; a pushrod that's bent, seized, or missing its clevis pin; or any other hardware in the actuation path that isn't doing its job correctly. The regulation doesn't require a complete brake failure — a component that is degraded enough to be considered non-functional is sufficient for a citation.
This is a mechanical safety issue at the most fundamental level of your braking system. Inspectors are trained to look at these components closely during Level I and Level II inspections, and because the defect is often visible or audible, it tends to get caught when it exists.
What our enforcement data actually shows
Across our database of 13 million+ inspections, 393.47(b) has accumulated 3,181 all-time citations, placing it at #427 out of 3,036 FMCSR codes by citation volume. That's a mid-tier enforcement footprint — common enough to take seriously, but not one of the most frequently written codes on the road.
What stands out sharply is the out-of-service rate. Of those 3,181 citations, 2,466 resulted in the vehicle being placed out of service — that's a 77.5% OOS rate. To put that in perspective, the all-FMCSR average OOS rate across every code in our database is 31.4%. The rate for 393.47(b) is more than double that average. Even though the code is technically marked OOS-eligible as "no" in the regulatory framework, our inspection records show that inspectors are placing vehicles out of service under this citation at an extraordinarily high rate — likely because when a brake actuator is genuinely defective, the vehicle presents an immediate danger and an out-of-service order is applied under related brake OOS criteria.
In the last 12 months, our data shows 0 citations, and in the last 90 days, 0 citations. This suggests enforcement activity under this specific sub-code has gone quiet in recent periods, but the all-time record — and especially that 77.5% OOS rate — means you cannot treat a citation as a minor administrative matter.
Who gets cited most
The top vehicle makes cited under 393.47(b) in our database tell a clear story about where this defect shows up. Freightliner (FRHT) leads with 467 citations, followed by Peterbilt (PTRB) at 192 and Freightlin at 186 — with Kenworth (KW) close behind at 183. These are among the most common heavy-duty platforms on American roads, so their presence at the top of the list reflects their market share as much as any specific mechanical vulnerability. International (INTL) accounts for 117 citations, and Utility (UTIL) trailers appear at 96, confirming that trailer axle brake chambers are a real inspection target — not just the tractor.
On the carrier side, our data shows fleets such as EVANS DELIVERY COMPANY INC (USDOT 38111) with 8 citations and TRANSPORTES AGUILA DE CIUDAD JUAREZ SA DE CV (USDOT 555365) also with 8 citations appearing at the top of the all-time carrier list. TEXAS INTERNATIONAL ENTERPRISES INC (USDOT 2196912) follows with 7 citations. These numbers reflect long-term accumulation and are not a judgment on current operations.
Note: The statistics block for this code does not include a state-by-state breakdown, so no state-level comparison is presented here.
How severe is this compared to similar codes
To understand where 393.47(b) sits in the landscape of Vehicle Maintenance violations, compare it to a few peer codes from the same category in our database.
393.9(a) — Inoperable required lamps has been cited 660,737 times with a 15.4% OOS rate. That's more than 200 times the citation volume of 393.47(b), but the OOS rate is a fraction of it. Lamps get written up constantly; defective brake actuators send trucks to the shoulder.
396.3(a)(1) — Inspection, repair, and maintenance (general) shows 236,919 citations and a 45.3% OOS rate. That's a significantly higher volume than 393.47(b) and a meaningful OOS rate — but still well below the 77.5% rate attached to defective brake actuators.
393.47E — Slack adjuster defective has 180,363 citations and a 0.0% OOS rate in our database. That's a related brake component code, and the contrast is striking: slack adjuster defects at 0.0% OOS versus brake actuator defects at 77.5%. The data tells you that inspectors view a defective actuator or chamber as a far more serious immediate threat than an out-of-adjustment slack adjuster.
The takeaway for fleet managers: 393.47(b) punches far above its citation-volume weight when it comes to operational disruption. A single citation carries a roughly 3-in-4 chance of putting your truck out of service on the spot.
How to avoid it
Because our inspection records show brake actuator defects appearing across nearly every major truck and trailer platform, prevention has to be baked into your pre-trip routine — not left to shop intervals alone. Here's what you can do before you leave the yard:
- Walk every brake chamber on the tractor and trailer. Look for cracks in the chamber body, corrosion around the clamp ring, and any sign of the diaphragm pushing air out where it shouldn't. On Utility and other common trailer makes, these chambers sit exposed and are easy to visually check.
- Listen for air leaks with the system charged. A leaking chamber diaphragm will often be audible at or near the wheel end. If you hear hissing at a brake location, do not move the vehicle.
- Check pushrod condition and travel. With the brakes applied, verify that the pushrod extends smoothly and retracts when released. Bent or seized pushrods are a direct path to a 393.47(b) citation.
- Inspect clevis pins and cotter pins. These small connectors between the pushrod and the slack adjuster are failure points. A missing or sheared cotter pin means the connection can walk out under application.
- On Freightliner, Peterbilt, and Kenworth platforms specifically, pay attention to rear drive axle chambers — our citation data shows these makes account for the largest share of 393.47(b) citations, and rear axle chambers on heavy-haul configurations are among the first to show heat and fatigue damage.
- Report any brake system abnormality on your DVIR. If something felt soft, spongy, or delayed on your last run, document it. A mechanic catching a failing diaphragm before the next inspection is the only outcome that doesn't involve an OOS order and a CSA severity weight of 7 attached to your record.