Hydraulic Brake Failures

Hydraulic Brakes in Commercial Vehicles

Hydraulic brake failures do not usually involve the 80,000-pound tractor-trailer most people picture when they hear “truck crash.” They often involve medium-duty vehicles: box trucks, cutaway delivery vans, shuttle buses, school buses, rental trucks, utility vehicles, and other commercial vehicles that may fall below the size and weight of a typical air-braked tractor-trailer combination. Federal regulations recognize hydraulic brakes as commercial-vehicle braking systems. Under 49 C.F.R. Part 393, commercial motor vehicles must have brakes adequate to stop and hold the vehicle, and commercial vehicles with hydraulic brake systems manufactured on or after September 2, 1983 must meet Federal Motor Vehicle Safety Standard No. 105.¹

That distinction matters because hydraulic brakes fail differently from air brakes. A hydraulic brake system uses fluid pressure to transmit force from the driver’s pedal to the wheel-end brakes. FMVSS No. 105 defines a hydraulic brake system as one that uses hydraulic fluid as the medium for transmitting force from the brake control to the service brakes. The same standard applies to trucks and buses equipped with hydraulic or electric brake systems within the covered vehicle classes.²

The risk is not theoretical. NHTSA’s 2023 large-truck fact sheet defines large trucks as medium or heavy trucks with a gross vehicle weight rating greater than 10,000 pounds, excluding buses and motor homes. In 2023, an estimated 528,177 large trucks were involved in police-reported crashes, and medium trucks between 10,001 and 26,000 pounds accounted for 29 percent of large trucks in fatal crashes and 39 percent of large trucks in injury crashes.³

Hydraulic-brake vehicles are often used in local delivery, school transportation, passenger shuttle work, construction support, and rental fleets. That means they may be driven by people who are not full-time heavy-truck drivers or who are operating an unfamiliar vehicle on an unfamiliar route. In the Large Truck Crash Causation Study, FMCSA reported that “unfamiliar with roadway” was coded for an estimated 30,000 large trucks, or 21.6 percent, and “unfamiliar with vehicle” for 9,000 large trucks, or 6.5 percent, among associated factors in the study’s crash population.⁴

Those statistics are not hydraulic-brake-specific, but they are useful because hydraulic brake failures often depend on recognition. A driver may need to understand what a sinking pedal, soft pedal, brake odor, longer stopping distance, or downgrade braking problem means before the system loses usable stopping power.

How Hydraulic Brake Failure Happens

A properly working hydraulic brake system depends on pressure. When the driver presses the brake pedal, the master cylinder must build hydraulic pressure and transmit that pressure through brake lines and hoses to calipers or wheel cylinders. The wheel-end components then press pads or shoes against rotors or drums, converting vehicle motion into heat. If the system cannot build or maintain pressure, the brake pedal may feel soft, sink toward the floor, or fail to create enough braking force.

The FMCSA Commercial Driver’s License Manual explains hydraulic brake failure in practical terms. It states that most hydraulic brake failures occur for one of two reasons: loss of hydraulic pressure or brake fade on long hills. The manual identifies a “spongy” pedal or a pedal that goes to the floor as signs that the hydraulic system may have lost pressure.⁵

The failure may begin with a leak.

Brake fluid can escape from a:

Air can enter the system after a leak or after improper service. A line may rub against a frame bracket, body mount, steering shaft, axle component, or exhaust pipe until it wears through. Federal regulations directly address those risks: brake tubing and hose must be protected from chafing, kinking, mechanical damage, and contact with high-temperature sources.⁶

Heat creates a separate failure path. Braking converts speed into heat, and repeated braking on a long downgrade can overheat the system. The CDL manual warns that excessive use of service brakes can cause brake fade, reducing stopping power, and instructs drivers to select a safe speed and use proper braking technique before starting a long downgrade.⁷ In hydraulic systems, heat matters because brake fluid has a boiling point. FMVSS No. 116 sets minimum dry and wet equilibrium reflux boiling points for motor vehicle brake fluids. DOT 3 brake fluid must have a dry boiling point of at least 205°C, or 401°F, and a wet boiling point of at least 140°C, or 284°F. DOT 4 must meet 230°C, or 446°F, dry, and 155°C, or 311°F, wet.⁸

The difference between dry and wet boiling point is critical. Brake fluid can absorb moisture over time. Once moisture contamination lowers the boiling margin, repeated braking can produce vapor in the hydraulic system. Vapor compresses more than liquid, so the driver may experience a soft pedal, excessive pedal travel, or a temporary loss of effective braking. A TRB record for an SAE paper titled Brake Fluid Vaporization as a Contributing Factor in Motor Vehicle Collisions describes brake-fluid vaporization as a crash-reconstruction issue and discusses testing involving brake fluid with different moisture-contamination levels.⁹

What the Crash Data Shows—and What It Misses

Brake problems appear repeatedly in national crash and inspection data, but the numbers must be used carefully. The Large Truck Crash Causation Study did not isolate hydraulic brake systems. It examined serious crashes involving large trucks and coded “Brake Failure, out of adjustment, etc.” as an associated factor for an estimated 41,000 trucks, or 29.4 percent of the study population.¹⁰

The study’s methodology gives that number weight. FMCSA’s analysis brief explains that the LTCCS sample included 963 crashes involving 1,123 large trucks and 959 non-large-truck motor vehicles, resulting in 249 fatalities and 1,654 injuries. Crash researchers and state truck inspectors collected scene data, inspected trucks, interviewed drivers and witnesses, reviewed logbooks and documentation, and collected up to 1,000 data elements per crash.¹¹

NHTSA’s National Motor Vehicle Crash Causation Survey offers another useful caution. In that survey, the critical reason was assigned to vehicle failure or degradation in about 2 percent of crashes, and brake-related problems accounted for about 22 percent of those vehicle-related critical reasons. But NHTSA also warned that vehicle-related critical reasons were mostly inferred from external visual inspection because the survey did not involve detailed mechanical inspections.¹² Internal brake problems may therefore be underrepresented.

Roadside inspection data also shows that brake defects remain a recurring commercial-vehicle enforcement problem. During 2025 Brake Safety Week, CVSA reported 15,175 brake-focused inspections across North America and 2,296 brake-related out-of-service violations, a 15.1 percent brake-related out-of-service rate.¹³ CVSA’s broader Operation Airbrake results show the same recurring pattern across multiple years. Brake-focused inspection campaigns regularly remove a meaningful share of inspected commercial vehicles from service for brake-related violations.¹⁴

Those enforcement numbers are not limited to hydraulic brakes. Many involve air-braked tractor-trailers. But they show the larger oversight problem: commercial brake defects are common enough to require recurring national enforcement campaigns. When the vehicle is a cutaway truck, school bus, shuttle bus, or medium-duty commercial vehicle with hydraulic brakes, investigators should not assume the brake system was functioning simply because the vehicle was smaller than a tractor-trailer.

Gray Summit: A Hydraulic Brake Failure Investigation in Detail

The NTSB’s investigation of the August 5, 2010 Gray Summit, Missouri school bus crash shows how hydraulic brake evidence can appear after a collision. When investigators depressed the brake pedal on the lead bus, the pedal movement was soft and spongy, and a brake-fluid leak was observed in lines running from the master cylinder to the ABS unit. During later evaluation, the brake-fluid reservoir was empty down to the fluid intakes at the bottom of both chambers, and no braking force was available.¹⁵

The physical evidence was specific. The brake lines were sent to the NTSB Materials Laboratory, where investigators found both lines corroded and one line with a 0.06-by-0.03-inch hole.¹⁶ Testing showed how much the defect mattered. NTSB deceleration testing estimated that at 50 mph, the lead school bus with its damaged brake line would need 9.13 seconds and 335 feet to stop. With the brake lines repaired, the same bus would need 3.60 seconds and 132 feet. The following bus would need 3.55 seconds and 130 feet.¹⁷

Those numbers are important because they show why hydraulic brake failures can be hidden inside a simple crash description. A police report may describe a rear-end crash or a driver who failed to stop. A mechanical investigation may show that the vehicle needed more than twice the stopping distance because the hydraulic system could not build pressure.

The maintenance evidence was just as important. NTSB found that the carrier had no written vehicle maintenance plan and no organized method for retaining maintenance records. The entire set of maintenance records for a 23-bus fleet was kept in a small spiral-bound notebook, and some service checks, such as topping off fluid, were not recorded. NTSB concluded that the condition of the brakes "was a consequence of the lack of scheduled maintenance."¹⁸ NTSB also identified inspection failures. The level of corrosion, the size of the hole in the brake line, and paint degradation near the line suggested the lines might have been leaking for months before the crash. Yet the leak was not found during an inspection of the fleet months earlier or during a private inspection 10 days before the crash.¹⁹

The NTSB safety recommendation letter following Gray Summit made the oversight point directly. NTSB stated that if Missouri’s inspection form had prompted inspectors to check brake lines for air and fluid leaks, the lead bus’s brake defects might have been detected months before the crash. The letter also noted that after the crash, Missouri inspected the operator’s fleet and rejected 8 of 20 buses due to defects, including defective and inoperable brakes.²⁰

A NTSB Materials Laboratory report reinforces the evidence strategy. Investigators submitted three brake-fluid samples for contaminant testing: two from the accident bus and one from another bus with an intact brake system.²¹ The accident-bus line sample showed 2 percent oil contamination, the master-cylinder sample showed 1 percent oil contamination, and the third-party laboratory reported possible oil contamination for those samples.²²

Schoharie: When Poor Maintenance and Grades Combine

The NTSB’s Schoharie, New York limousine crash report provides another detailed example of hydraulic brake failure under grade conditions. The vehicle was a heavily altered Ford Excursion stretch limousine. NTSB explained the hydraulic system in ordinary mechanical terms: pressing the brake pedal moves the master-cylinder pistons, pushes fluid into pressure chambers, sends fluid through brake lines to calipers, and forces brake pads against spinning rotors, creating heat and braking torque.²³

The inspection findings were severe. NTSB observed a corroded steel brake line attached near the right rear differential housing, coating consistent with brake fluid, and a crimp near the connection to the right rear flexible brake hose. NTSB stated that the leak appeared likely to be a preexisting condition rather than crash damage and that loss of integrity in the line near the right rear brake assembly could result in rear brake failure due to loss of hydraulic pressure.²⁴

The wheel-end evidence also mattered. NTSB found that the right rear caliper was heavily corroded and that both pistons appeared seized in place. Seized pistons would be unable to apply hydraulic force to the brake pad, potentially making that brake assembly inoperable. The front brakes showed signs of high temperature, including burned-material odor and embedded friction material.²⁵

The route gave those defects operational significance. NTSB noted that the trip included rolling terrain and a 2.3-mile descending grade with an elevation decrease of more than 700 feet earlier in the trip. The crash occurred as the limousine descended toward an intersection, and NTSB determined that the limousine was unable to slow on the downhill grade because of a failure within a poorly maintained brake system.²⁶

Schoharie should not be reduced to a single-cause “brake fluid boiling” example. The NTSB findings were broader. Hydraulic pressure loss, corrosion, seized rear calipers, overheated front brakes, poor maintenance, and grade demands all interacted. That is exactly why hydraulic brake investigations require more than checking whether the pedal moves after the crash.

Recalls Show the Same Failure Patterns in the Field

NHTSA recall records show that hydraulic brake problems repeatedly involve routing, clearance, heat, rubbing, and leaks. In a 2024 GM recall involving Chevrolet Express and GMC Savana cutaway vehicles, NHTSA reported that brake lines in commercial cutaway vehicles with GM-provided body mounts could lack adequate clearance. If the lines contacted the mounts, wear or damage could lead to a brake-fluid leak, reduced stopping performance, or inoperative service brakes.²⁷

The same recall chronology identified 18 Transport Canada incidents involving brake-line wear or damage, including seven involving fluid loss and five involving pedal feel or degraded brake performance.²⁸ A 2026 IC Bus recall involved a brake-line retainer that could allow the brake line to disengage and contact the steering shaft. NHTSA’s recall report stated that contact between the brake line and steering shaft may result in a hydraulic leak, soft pedal, increased stopping distance in an emergency stop, and possible crash risk.²⁹

Blue Bird recall records show similar risks. A 2021 recall reported that a missing retaining clamp could allow a hydraulic brake line near the engine to rub on an exhaust pipe, causing a leak or line failure. The report stated that failure could lead to a low-brake-pressure warning and eventual loss or degradation of front and rear service brakes.³⁰ Another Blue Bird recall, issued in 2018, involved hydraulic brake lines routed near the exhaust manifold on certain Vision buses. The report stated that excessive heat could cause premature failure and brake fluid leaking onto the manifold; the chronology described a dealer report in which flexible hydraulic brake lines failed and leaked brake fluid onto the exhaust manifold, resulting in a thermal event.³¹

Medium-duty trucks have also been subject to hydraulic-fluid leak recalls. A 2025 NHTSA recall involving certain Chevrolet Silverado Medium Duty 4500HD, 5500HD, and 6500HD vehicles described a brake pressure sensor assembly that could allow brake fluid to leak beyond a diaphragm seal into the brake pressure switch.³²

These recall records are useful because they show concrete failure pathways. A hydraulic brake failure may begin with a missing clamp, poor body-mount clearance, a line routed too close to exhaust heat, a hose stretched by axle movement, a corroded line, or a repair that leaves air in the system. A Florida Department of Education recall notice for school buses described hydraulic brake drop hoses that were too short and could fail from axle-articulation tension, causing loss of rear brakes; the remedy was to replace the hoses and bleed the brake system.³³

What Investigators Should Look For After a Hydraulic Brake Crash

Hydraulic brake investigations should begin before the vehicle is repaired, moved again, or scrapped.

NHTSA’s Special Crash Investigations program describes in-depth investigations that can include scene inspection, vehicle inspection, interviews, event data recorder information when available, photographs, police reports, medical records, and other crash-specific evidence.³⁴

The first evidence question is pressure. Did the pedal feel firm, spongy, or dead? Did it sink under steady pressure? Was the reservoir full before testing? Did fluid leak during pedal application? Were there wet areas on the master cylinder, ABS module, frame rail, flexible hoses, wheel ends, or pavement? The CDL manual’s hydraulic-brake inspection procedure tells drivers to pump the brake pedal three times, apply firm pressure for five seconds, and treat pedal movement as evidence of a leak or other problem that must be fixed before driving.³⁵

The second question is heat. Investigators should document grade length, vehicle weight, load, route choice, driver familiarity, brake odor, rotor condition, pad appearance, and whether the driver used engine braking. FMVSS No. 105 includes fade and recovery testing requirements, reflecting that hydraulic brake performance must be evaluated not only in a single stop, but after repeated heat-generating applications.³⁶

The third question is maintenance. Federal rules require motor carriers to systematically inspect, repair, and maintain all motor vehicles under their control.³⁷ For a hydraulic-brake vehicle, that means investigators should request service files, brake-fluid records, line and hose repairs, caliper replacements, master-cylinder work, ABS repairs, recall completion records, driver complaints, inspection reports, and proof that the system was properly bled after hydraulic service.

The fourth question is whether the crash description hides a mechanical failure. A report may say the driver failed to stop, followed too closely, or lost control. Those observations may be accurate, but incomplete. NHTSA’s NMVCCS warning is important: vehicle-related critical reasons were mostly based on external visual inspection, so internal vehicle problems may not be fully represented in broad crash statistics.

Hydraulic brake failures are often reconstructable because they leave evidence in fluid, metal, heat, routing, service records, inspection records, and pedal behavior. A complete investigation asks whether the vehicle could build pressure, whether brake fluid leaked or boiled, whether heat exceeded the system’s margin, whether the driver had reason to recognize the problem, and whether the maintenance history shows a sudden failure or a defect allowed to develop over time.

Sources

• [1] Federal Motor Carrier Safety Administration, 49 C.F.R. Part 393, Subpart C (Brakes).
• [2] National Highway Traffic Safety Administration, 49 C.F.R. § 571.105 (FMVSS No. 105, Hydraulic and Electric Brake Systems).
• [3] National Highway Traffic Safety Administration, "Large Trucks: 2023 Data" (Traffic Safety Facts, Report No. DOT HS 813 717, April 2025).
• [4] Federal Motor Carrier Safety Administration, "Report to Congress on the Large Truck Crash Causation Study" (MC-R/MC-RRA, March 2006).
• [5] Federal Motor Carrier Safety Administration, Commercial Driver's License Manual (2005 Edition, Revised July 2014).
• [6] Federal Motor Carrier Safety Administration, 49 C.F.R. Part 393, Subpart C (Brakes), supra note 1.
• [7] Federal Motor Carrier Safety Administration, Commercial Driver's License Manual, supra note 5.
• [8] National Highway Traffic Safety Administration, 49 C.F.R. § 571.116 (FMVSS No. 116, Motor Vehicle Brake Fluids).
• [9] Transportation Research Board, "Brake Fluid Vaporization as a Contributing Factor in Motor Vehicle Collisions" (SAE paper, TRB record).
• [10] Federal Motor Carrier Safety Administration, "Report to Congress on the Large Truck Crash Causation Study," supra note 4.
• [11] Federal Motor Carrier Safety Administration, "The Large Truck Crash Causation Study - Analysis Brief" (Publication No. FMCSA-RRA-07-017, July 2007).
• [12] National Highway Traffic Safety Administration, "Critical Reasons for Crashes Investigated in the National Motor Vehicle Crash Causation Survey" (Traffic Safety Facts Crash•Stats, Report No. DOT HS 812 115, February 2015).
• [13] Commercial Vehicle Safety Alliance, "2025 Brake Safety Week Results" (2025).
• [14] Commercial Vehicle Safety Alliance, "Operation Airbrake: Brake Safety Campaign Results".
• [15] National Transportation Safety Board, Highway Accident Report NTSB/HAR-11/03, "Multivehicle Collision, Interstate 44 Eastbound, Gray Summit, Missouri, August 5, 2010".
• [16] Id.
• [17] Id.
• [18] Id.
• [19] Id.
• [20] National Transportation Safety Board, Safety Recommendation Letter H-11-040 through -045.
• [21] Arnold & Itkin LLP, Materials Laboratory Factual Report No. 11-003 (Brake Fluid Testing), NTSB Docket No. HWY10MH018 (Gray Summit, Missouri investigation) (obtained from the NTSB public docket and on file with the firm; not independently accessible via a stable public URL).
• [22] Id.
• [23] National Transportation Safety Board, Highway Accident Report NTSB/HAR-20/03, "Roadway Departure and Collision with Culvert Headwall, Stretch Limousine, Schoharie, New York, October 6, 2018".
• [24] Id.
• [25] Id.
• [26] Id.
• [27] National Highway Traffic Safety Administration, Recall Report No. 24V702, Chevrolet Express / GMC Savana Cutaway Vehicles (2024).
• [28] Id.
• [29] National Highway Traffic Safety Administration, Recall Report No. 26V111, IC Bus (2026).
• [30] National Highway Traffic Safety Administration, Recall Report No. 21V778, Blue Bird (2021).
• [31] National Highway Traffic Safety Administration, Recall Report No. 18V885, Blue Bird Vision (2018).
• [32] National Highway Traffic Safety Administration, Recall Report No. 25V390, Chevrolet Silverado Medium Duty 4500HD/5500HD/6500HD (2025).
• [33] Florida Department of Education, School Bus Recall Notice T05-01.
• [34] National Highway Traffic Safety Administration, "Special Crash Investigations (SCI)".
• [35] Federal Motor Carrier Safety Administration, Commercial Driver's License Manual, supra note 5.
• [36] National Highway Traffic Safety Administration, 49 C.F.R. § 571.105 (FMVSS No. 105, Hydraulic and Electric Brake Systems), supra note 2.
• [37] Federal Motor Carrier Safety Administration, 49 C.F.R. § 396.3 (Inspection, Repair, and Maintenance).