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Air Brakes or Hydraulic Brakes: That is the Question

Choosing the right brakes for a medium-duty truck can make spec’ing complex; however, there are clear-cut reasons to use both, depending on payload and work environment.

March 2012, Work Truck - Feature

by Sean Lyden - Also by this author

Air Brakes

How They Work: Instead of using fluid, air brakes, as the name implies, use air to generate stopping power. When the air tanks are fully pressurized, the brakes are disengaged. When the driver presses the brake pedal, air fills the brake chamber, pushing the chamber diaphragm, which turns the "S-cam," and then pushes the brake pads against the brake drum, stopping the vehicle. Then, when the brake pedal retracts, the air is released allowing the brakes to release and the wheels to roll. The compressor increases air pressure back to the system's original state.

Truck Size: 26,000-33,000 lbs. and larger. "While hydraulic brakes are standard on our Class 5 and 6 vehicles and air brakes on Class 7 and greater, we do allow a crossover where air brakes can be spec'd on lighter vehicles [under 33,000-lbs. GVWR]," Moore said.

Applications: Moore recommends air brakes for heavy vocational applications and noted they should always be used in heavy towing applications.

A significant reason why air brakes are preferred in heavier trucks (above 26,000-lbs. GVWR), compared to hydraulic systems, is their robust stopping power when they work - and when they fail. For example, if there's a leak in the brake line of an hydraulic system, fluid pressure can lower to the point where there isn't sufficient force on the brake pads to create the friction needed to slow the wheel. Eventually, if the leak is not repaired, the truck can lose braking power in that portion of the system, reducing the ability to stop in the same distance. With air brakes, the opposite happens. If there is a leak in the air brake lines, the air pressure decreases, which actually activates the brakes at the wheels and brings the vehicle to a safe stop.

However, air brakes come at a premium price. According to Kaufman of Ford, the air brake system costs approximately $2,500 more than hydraulic brakes, because of the extra components to operate the system. "When you compress air, you have moisture, and you have to get rid of that moisture so you're adding air dryers as part of the initial purchase. But, if you're going to keep the vehicle for more than five years - maintenance costs tend to go more vertical after year five and get really expensive. After that, I think air brakes pay for themselves," he said.

Another factor with air brakes is how they impact a fleet's available driver pool. Even if the truck is under 26,000-lbs. GVWR, which would normally not require a commercial driver's license (CDL) to operate, if it is equipped with air brakes, the driver may have to carry a CDL, depending on the state's laws, which limits the number of drivers qualified to operate the truck.

"Air brakes, for lack of a better description, are either 'on' or they are 'off.' If you've never driven an air brake truck, the first few times you press the brake, you feel like you're putting yourself through the windshield. Unlike hydraulic brakes, which modulate more intuitively, the operator has a lot to do with actively modulating the air brakes to make the stopping process smoother. That's something the driver learns," Kaufman noted.

Other Considerations

In addition to the hydraulic versus air brake decision, there are other braking technologies fleet managers should familiarize themselves with as they specify medium-duty trucks. These include:

■ Anti-Lock Braking Systems (ABS). ABS is standard for most medium-duty truck manufacturers for both hydraulic and air brakes. ABS automatically adjusts pressure to the brakes during severe braking to prevent wheel lockup, helping a driver maintain control of the vehicle. Most insurance companies offer discounts for trucks that are ABS equipped.

■ Exhaust Brakes. Available only with diesel engines, the exhaust brake closes off the exhaust manifold from the engine, creating backpressure in the engine cylinders, slowing the engine's pistons and, eventually, the vehicle as a whole. Since the exhaust brake works off the engine and not the wheels, it helps prolong the life of both hydraulic and air brake systems.

The Bottom Line

With medium-duty trucks in the 26,000-33,000-lb. range, there is much to consider when spec'ing brakes. When vehicle duty cycle, cost-benefit analysis of air brakes, and CDL driver availability are factored, valuable insight will be gained regarding the optimal brake system for the vehicle and fleet operations. WT


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Comments

  1. 1. John alves [ June 17, 2013 @ 04:05PM ]

    Can I get a 30 ft box truck that is hydraulic over air brakes and not need a cdl my friend said he thought he saw something like that

  2. 2. Richard Steele [ September 26, 2015 @ 05:16AM ]

    "Air brakes are either on or off". That is the silliest statement I have ever read.

  3. 3. Xman [ October 06, 2016 @ 10:19AM ]

    Is it not the size of the brakes that decide the stopping ability, not whether they are hydraulic or air? Regardless equally "sized" brakes, air or hydraulic are equal and equal sized drums and disc, disc is always better.

  4. 4. Michael Reilly [ April 25, 2017 @ 06:12PM ]

    I drove a 33,000lb box truck today with air brakes and I could immediately feel the difference between the air brakes and hydraulic brakes. The immediate stopping power is much more prominent in air brakes than hydraulic brakes and you can feel and hear the air jerk when you hit on the brakes. It's pretty impressive engineering and must have been designed by a genius.

  5. 5. Ray White [ July 05, 2017 @ 06:40PM ]

    I don't know [do you?] if Florida requires a CDL to drive a truck with air brakes, if it's under 26,000.
    I caution anyone considering purchasing any truck having a "Lucas-Girling" brake system. It is hydraulic, expensive, and maintenance-intensive!

 

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