What 393.43(b) means in plain language
FMCSR 393.43(b) addresses the relay valve and emergency valve that are part of your truck's air brake system. These components work together to control how air pressure flows to your service brakes and parking brakes. When an inspector cites you for this violation, they've found that one of these valves is defective or not functioning correctly.
A defective relay or emergency valve can cause inconsistent brake response, delayed brake application, or failure of the parking brake to hold properly. The valve may leak air, fail to respond to pressure signals, or get stuck in a position that prevents normal brake operation. This is not a minor issue—your brakes are your primary safety tool on the road.
What our enforcement data actually shows
Across 13 million inspections in our database, 393.43(b) has generated 280 all-time citations, making it ranked #1112 of 3,036 FMCSR codes by citation volume. What's most important: the OOS rate for this code is 0.0%—none of the 280 citations resulted in an out-of-service order.
This is significantly lower than the all-FMCSR average OOS rate of 31.4%, indicating that inspectors typically do not immediately remove vehicles from service for this violation. However, this does not mean the defect is acceptable. Over the last 12 months, we have recorded 0 citations for this code, and similarly 0 citations in the last 90 days, suggesting either improved compliance or reduced enforcement activity for this specific violation.
The low enforcement volume and zero recent citations should not be interpreted as permission to ignore brake system maintenance. Rather, it reflects that this particular valve defect may be caught less frequently than other brake-related violations, or operators may have improved their maintenance protocols.
Who gets cited most
Our inspection records show that brake relay and emergency valve violations are concentrated among a small number of carriers. AUTOTRANSPORTES ROMEDU SA DE CV (USDOT 1148259) leads with 12 citations across our database. CLAUDIA BELINDA ANDARZA LEAL (USDOT 963435) follows with 9 citations. These patterns indicate that certain fleets or owner-operators have experienced repeated citations for this defect.
The vehicle makes most frequently cited for 393.43(b) violations include Freightliner (82 citations), International (77 citations), and Kenworth (44 citations). This distribution reflects the prevalence of these heavy-duty manufacturers on the road rather than a specific design flaw—the defect is maintenance-related, not inherent to any particular make.
How severe is this compared to similar codes
Compare 393.43(b) to other brake and maintenance violations in the same category. The brake slack adjuster code (393.47E) has generated 180,363 citations with a 0.0% OOS rate, far exceeding 393.43(b)'s volume. The general inspection and repair code (396.3(a)(1)) has 236,919 citations and a 45.3% OOS rate, indicating that when broader maintenance defects are cited, they're more likely to result in out-of-service orders.
Lighting violations, which also fall into vehicle maintenance, show much higher citation volumes: inoperable required lamps (393.9(a)) account for 660,737 citations with a 15.4% OOS rate. This suggests that while relay and emergency valve defects are less frequently cited than lighting issues, when they are found, they're treated with more leniency regarding immediate removal from service—though they still require correction.
How to avoid it
Perform thorough pre-trip brake system inspections. Do not skip the air brake components. Locate the relay and emergency valves on your unit and listen for leaks or unusual sounds. Press the brake pedal firmly and listen for a consistent hiss—hesitation or irregular sounds suggest a valve malfunction.
Check air pressure buildup during startup. When you first start your engine, the air compressor should build pressure smoothly. If pressure climbs slowly, fluctuates, or fails to reach normal operating range (typically 100–125 psi), a relay or emergency valve may be leaking or stuck.
Inspect air lines and connections to the valve. Loose fittings, cracked tubing, or visible corrosion around the valve ports can indicate a failing component. Tighten any loose connections and report damaged lines to maintenance immediately.
Test the parking brake hold. Apply the parking brake firmly and feel for a solid, unyielding response. If the brake feels soft or gradually releases, the emergency valve may not be sealing properly. This is a safety red flag.
Have your air brake system serviced annually by a qualified technician. Relay and emergency valves are not wear items that drivers can rebuild on the side of the road. Professional service ensures all internal seals and moving parts are clean and functional. This is especially important for Freightliner and International units, which appear most frequently in our citation data.
Document your maintenance records. Keep receipts and service records for all brake system work. If you are cited, these records demonstrate that you are actively maintaining the system and may provide context if the defect was recent or newly developed.