Pepsi or Coke, cats or dogs, air disc or air drum brakes. The things we all probably feel strongly about — or at least those who work in a garage for the last part, specifically.

Whether your school buses run with air disc, air drum, or hydraulic, you’re probably loyal to the system you know and trust. For most, that means the traditional air drum brakes. But some say it might be time to consider a change (if your state allows), as air disc systems gain popularity.

One of those is Shane Cook, fleet manager for Wilson County Schools in Tennessee. “We made the switch to air disc brakes in 2017 and have not looked back,” he told us. So, we asked more to learn what led to the change of heart.

But first, the basics.

A Quick Comparison of the Most Common School Bus Brake Systems

Air Drum Brakes

Long considered the standard for heavy-duty vehicles.

• How they work: Compressed air pushes brake shoes outward against a drum to slow the bus.
• Pros: Simple and widely known among mechanics. Easy parts availability. Durable, cost-effective, and provides proven braking power.
• Cons: Prone to fade and more maintenance. Very heavy drums.
• Best for: Type C and D buses on established routes with predictable traffic and flat to gently rolling terrain.

Air Disc Brakes (ADBs)

Recently gaining more popularity.

• How they work: Compressed air squeezes brake pads against a rotor.
• Pros: Better cooling and consistent stopping power, easier maintenance.
• Cons: Higher priced upfront, more specialized parts needs, and technicians may require more training.
• Best for: Type C and D buses on hilly, high-speed, and/or longer-mileage routes and heavier loads.

Hydraulic Brakes

Used more often on smaller vehicles with lighter loads.

• How they work: Compressed brake fluid (not air) squeezes brake pads against a rotor.
• Pros: Similar feel to automotive brakes, relatively easy maintenance, no special CDL certification for air brakes.
• Cons: Requires fluid to function, prone to rust.
• Best for: Type A buses or MFSABs operating at lower speeds with frequent stops.

Brakes of Choice Across Bus Fleets

While air drums have been king for decades, there is a slow shift happening.

We already mentioned that Wilson County School made the switch to air discs, as did Wisconsin-based contractor Kobussen Buses, for its new bus orders.

Meanwhile,  Summit School Services specs both hydraulic and air brake systems. The choice comes down to the location, driver and maintenance staff qualification profiles, operating parameters, and customer requirements.

Summit previously piloted air disc brakes, and the experience was mixed with significantly higher pad wear on the front disc rotor, Wayne Skinner, senior vice president, fleet, maintenance, & procurement, said. The company’s new Type C EVs (118 and counting) all have air disc brakes, which they plan to evaluate for reliability and cost savings.

For operators of electric school buses, air disc brakes’ lessened likelihood of brake fade can be a concern for EVs that rely on regenerative braking to recover energy. Air disc brakes remain the preferred option for Class 8 commercial battery-electric vehicles (BEVs), according to Bendix.

Maintenance Considerations

Air drums are a heavy lift — literally. An individual drum can clock in anywhere between 80 and 160 pounds. You’ve gotta be a physically strong person to handle those, Dan Kobussen commented, “and there seems to be fewer folks out there willing to do that,” he said, noting the higher potential for workers’ comp claims.

Air disc brakes bring their own considerations.

“The biggest maintenance mistake we see users make is not periodically inspecting the foundation brake,” Brian Screeton, manager of technical training and service at Bendix Commercial Vehicle Systems, said. “This lack of attention will let small issues turn into larger, more expensive issues, especially with air disc brakes.”

Jacob Lawless, fleet manager at Kobussen Buses, cites the drawback of frequent pad changes on air disc brakes. “New techs can miss the window to replace pads and cause costly damage to the rotor and/or caliper,” he warned.

While it may be harder to detect air disc brake wear, pad replacement is faster and easier. Bendix estimates it takes about one-quarter of the time compared to drum systems, with longer service intervals due to extended friction life and reduced component wear.

To ensure a seamless switch from drums to discs at Wilson County Schools, Cook made sure his technicians were properly trained on the differences in components, inspection, and repair. They brought in trainers from Bendix to deliver a class on the ADB22X system.

They also needed some new tools to do the job right. Cook’s team invested in the DBD-200, a disc brake dolly that handles both calipers and rotors to save some heavy lifting, as well as the X1HD brake lathe from Pro-Cut Solutions. The latter was used to turn rotors instead of replacing them each time. 

One benefit to the air disc over the air drum is that there is no brake stroke to check, Cook said, since the internal chamber is connected to the caliper. However, Bendix advises that while checking tire pressure (or every four months), techs should inspect the caliper running clearance, brake pad thickness, and rotor.

Hydraulic brake setups on Class 6 buses provide reliable performance, Skinner noted, though supplier variety at the OEM level results in different diagnostic tool requirements and repair procedures.

Over time, Summit did see a change in fluid viscosity, absorption of moisture, and in its stop-and-go cycle during pick-up and drop off. “It is imperative that brake fluid quality remains important, and replacement brake fluids used are of the appropriate type,” he said. “Rotors and pads are to be checked regularly during preventive maintenance inspection.”

All of Summit’s air brake systems now have automatic slack adjusters, and Skinner reminds that checking for proper function or replacing them promptly will ensure optimal performance.

Performance: Stopping Distance, Fade, and Control

For hybrid systems (when ADBs are on the steer axle and drum brakes are on the drive axle), stopping distance is shorter than it is on all-drum vehicles. If ADBs are on all axles, it will have the shortest stopping distance, according to Bendix.

Brake squeal and noise with their previous drum brake systems were a constant issue on Wilson County school buses. When they switched to air disc, complaints declined by over 90%. And the district’s drivers like the greater stopping distance since they feel more like brakes on passenger vehicles. 

Since brake fade is prevalent on drum brake systems from thermal expansion, there is greater running clearance. This means the drum brake must compensate for the increased clearance to initiate braking, resulting in longer stopping distances.

Lawless also compliments air disc’s stopping distance, noting it “greatly improved” by up to 40%, mostly gained in reduced brake fade.

Maintenance & Lifecycle Costs

While air disc brakes can be more costly up front, they can save in the long run.

“Our maintenance costs have gone down due to not having to purchase shoes and drums on air drums, to just replacing the pads and turning the rotors on the air disc,” Cook said.

Wilson County found that parts for the drum brake shoe kit and two brake drums averaged $250 to $300 each for replacement. Air disc pads, on average, range from $110 to $140 per axle. “We also save extra money by not having to replace the automatic slack adjusters and brake chambers that fail more often on drum brake systems,” he explained.

Lawless said that, so far, Kobussen Buses hasn’t seen cost savings with air discs. He reported that the rotors last longer, but parts are steeper, with calipers about 3-5 times more expensive. 

However, as Screeton points out, parts and labor have risen over the years across the board. Projections through 2028 indicate a 20% higher labor demand than expected, pushing wages upward as shops compete for a limited workforce.

Further, distributors and dealers report increases of 15% to 25% across truck parts, with brake drums seeing even steeper increases of 40% to 60%.

“From a Bendix brake perspective, our ADB22X air disc brakes are manufactured in Bowling Green, Kentucky, and are USMCA compliant for North American content,” Screeton said. “In general, rising labor costs continue to be influenced by the lingering effects of prior years’ high inflation.”

Driver Experience & Safety Features

According to Kobussen, the only drawback of air disc brakes is that they are spring-loaded, so if you lose air, you need an air pump driven off the engine. In most school buses, the pump is driven by a belt, and that belt can come off or wear out, leaving the bus dead in the water if you have no air. 

Beyond braking performance, air disc brakes at the wheel ends help optimize the performance of higher-level safety systems, Bendix said. And, they can play a role in supporting quiet, low-emission vehicle environments. “The brake’s consistent engagement and lower NVH (noise, vibration, and harshness) profile help reduce noise pollution and contribute to a smoother, more comfortable driving experience,” Rick Conklin, director of engineering, Wheel-End, at Bendix, said.

Plus, ADBs help mitigate accident risk from its shorter stopping distance performance.

In both air disc and air drum brake systems, emergency brakes are designed to lock on if air pressure is lost, providing an additional layer of safety.

Anti-lock braking systems (ABS) and electronic stability control (ESC) features have become standard across both systems. Make sure drivers fully understand the performance of these safety-assist features.

As collision mitigation systems gain acceptance, air disc brakes can enhance them by providing consistent, predictable braking performance, noted Bendix. Summit School Services appreciates this extra layer of safety, though they note not all locations have air brake-qualified/endorsed drivers.

When it comes to hydraulics, if there was a brake line failure, the system is designed to prioritize safety. Alarms alert the driver immediately, and the parking brake will engage once brake fluid is consumed. This mechanism ensures the vehicle comes to a complete, secure stop.

No matter what brake system is used, make sure drivers give them a check as part of their pre-trip. For hydraulic brake vehicles, that involves a visual inspection for fluid leaks, dash gauge notifications, and pedal feel.

The Future of School Bus Brakes

The industry appears to be slowly moving toward air disc brakes, and Keith McComsey, Bendix technical sales lead, Wheel-End, says thousands of new school buses are equipped with them each year. All major North American school bus manufacturers, including Blue Bird, IC Bus, and Thomas Built Buses, offer the Bendix ADB22X as a factory-installed option.

To evolve with the market as electric vehicles ramp up, Bendix is ready to support emerging needs, from compatibility with regenerative braking strategies to tighter integration with electronic control systems and diagnostics.

As much as Cook loves air discs, he wishes there was a better way of knowing when pads need replacement. He hopes to see a day when it’s standard for sensors to be installed on the brake pads to alert the driver via a dashboard when replacement is needed.

One of Skinner’s long-term brake wishes is for lower maintenance costs for air disc systems, especially since they can be prone to corrosion due to road salt. “As these systems improve, we would expect the transition to disc brakes would be more appropriate for school buses,” he said. (To add to that, he also mentions hope for a brake shoe life predictability indicator system to alert shop managers based on driver behavior, temperatures, prior repairs, or replacement timing).

“We hope that school systems continue to want safer braking systems adopted in their school bus specs, gravitating toward variations of air disc brakes, whether on steer only and/or on all axles,” McComsey said. “The more ADB axles there are, the shorter the stopping distance, and the more reliable/consistent the stops are, time after time.”

Regardless of which brake system makes your buses stop, remember that regular preventive maintenance remains crucial to peak performance and condition.