Air Brakes VS Hydraulic Brakes – 11 Facts You Need To Know

It doesn’t matter if you’re driving a VW Jetta or a big 18-wheeler. You expect the vehicle to slow down or stop when you hit that brake pedal.

This may be the end game in each braking system, but it’s important to know the difference between your car’s hydraulic brake system and that of a large RV or commercial vehicle equipped with air brakes.

Here are 11 facts about how hydraulic brakes and air brakes differ.

Are Air Brakes Better Than Hydraulic Brakes?

In heavy-duty applications, air brakes are simply the better option. Hydraulic brakes just couldn’t create the pressure required to stop an 80,000-pound truck and trailer and provide the flexibility needed to connect and disconnect to a variety of different trailers. 

Air Brakes Don’t Use Brake Fluid Like A Passenger Car

Okay. This should be obvious, but there’s no brake fluid in an air brake-equipped vehicle. The braking system is completely handled by compressed air that flows through the brake lines.

Another interesting difference between the two braking systems is what happens when you let your foot off the brake.

The brake fluid returns to the reservoir in your car or pickup’s hydraulic brake system. The system is sealed.

The compressed air is released into the atmosphere on air brakes. That’s why you hear the noise when a truck driver lifts his foot off the brakes.

What Type Of Vehicles Have Air Brakes?

A good rule of thumb is if it’s larger than a U-Haul truck, it probably has air brakes. Semi-trucks hauling one or more trailers all are equipped with air brakes. But there are a lot more vehicles out there that use them.

  • Heavy School Busses
  • Trains
  • Some larger motor homes
  • Larger fire fighting equipment like ladder trucks, tankers, and pump trucks
  • Many box trucks. Again, larger than a U-Haul or rated heavier.
  • Heavy equipment

Do Air Brakes Stop Faster Than Hydraulic Brakes

If all things were equal, the answer would still be no. It takes time for compressed air to fill a brake line and apply the brakes at the wheel.

Here’s what happens when a driver sees something requiring braking action.

  1. According to a report from the University of Idaho, the average response time from the moment the driver sees an event to when he lifts off the gas and applies the brakes is between .75 seconds and 3.5 seconds.
  2. Since hydraulic brake fluid hardly compresses at all, the brakes are applied instantly when the pedal is depressed. The fact that the fluid doesn’t have far to travel in even the largest cars is also a plus.
  3.  With air brakes, there’s at least another half-second lag time for the compressed air to reach the brake assembly at the vehicle and trailer wheels. Heep in mind, there’s at least 60 feet of air line for the compressed air to travel through.

What Are Air Brake Glad-Hands?

Glad-hands are the couplings used to connect the air brake lines of a semi-truck to a trailer. They’re also used to connect the second and third trailers into the truck air brake system.

These glad hands make connecting and disconnecting the trailer air lines easy.

Trains also use a glad hands to connect the engines much larger airline and train cars together. However, they’re larger in size.

Gladhands are designed to be easy to connect. The blue connection is for the service brake system, which responds when you hit the brake pedal. The red connection is for the parking brake.

Where Does The Air Come From For Air Brakes?

Air brake-equipped vehicles have a compressor that’s connected to the truck engine. This compressor will fill an air tank to around 110 to 120 psi. When service brakes are applied by hitting the brake pedal, the compressed air travels through the lines and does its thing.

The Compressed Air Does More Than Just Brakes

You’ve probably heard that most new semi-trucks are equipped with automatic transmissions, but that’s not exactly true. Rather than a traditional automatic transmission, nearly all of these are automated manual transmissions.

The inside of a big truck’s automated transmission operates like any manual truck transmission. It may have 10, 12, or more forward gears, but there’s no clutch pedal, and there’s no big shifter coming out of the floor.

Instead of a shifter, a computer tells the throttle, the clutch, and even the transmission exactly what to do and when. The computer uses servos that run off compressed air from the same compressor.

And don’t forget air ride suspension. The same compressor provides the air for the leveling and adjusting air ride-equipped trucks and trailers.

Air also controls features like the air-ride seats and air suspension for the cab and sleeper. And let’s not forget, features like power windows, heating and AC controls, even the air horn depend on the air compressor for power.

In older trucks, compressed air also ran power steering and windshield wipers.

Note: Some of the early automated transmissions did have clutch pedals. They were only used for starting and stopping. The computer handled all shifting chores.

Also, they are rare, but you will find a few big trucks with Allison automatic transmissions rather than the more common automated option.

What Happens When A Truck Breaks An Air Line?

First, in every car built since 1967, the US government required the braking system of all passenger cars to be separated into two separate hydraulic systems. The left-front wheel and right rear wheel are sealed from the other two wheels.

Air lines under a 53 foot semi-trailer
As you can see, there’s quite a bit of air line plumbed under the trailer.

The result is a safer vehicle. Should you damage or break a line, you’ll lose half of your stopping power, but the brakes will still function on the other two wheels.

In an air brake system, should you split or break an air line, and lose air to the point that the compressor can’t keep up, the system will start losing pressure. When it drops to a certain point, usually at about 30-40 psi, the parking/emergency will automatically come on.

This should be of no surprise to the driver. A loud audible alarm will have sounded at 70 pounds to warn them. The brakes on any connected trailer will also be set.

You’ll Need An Air Brake Endorsement

In nearly every application, you’ll need an air brake endorsement to operate any air brake-equipped vehicle, including a commercial truck.

In many states, RV drivers may be required to have airbrake endorsements. It depends on several factors, depending on the state.

Factors include gross vehicle weight, the total length of the vehicle, and whatever they’re towing behind. The weight of the towed vehicle or trailer also comes into play.

George Westinghouse Patented Air Brakes In 1852

That’s right! Air brakes were around long before cars or trucks. The air braking system designed and patented by Mr. Westinghouse was originally designed for use on trains.

Prior to the air brakes, a train would reduce power and sound the train whistle to alert the brakemen. The brakemen would then go from car to car and manually apply brakes by turning a brake wheel that was connected to the actual train wheel assembly.

Water Is Bad In Air Brake Lines

Water in an air brake line presents a couple of issues, one problem being corrosion over time. An even bigger problem is what happens when temperatures go below freezing. A frozen brake line or component can easily prevent air from flowing to and from the actual brake assembly at the wheel.

Air brake systems include air dryers, a device that removes all moisture from the air in the system. Unfortunately, the glad-hand system used to connect the airlines to a semi-trailer is vulnerable to moisture and snow buildup.

Do Semi-Trucks Have Anti-lock (ABS) Brakes?

Passenger cars and light trucks have had antilock brakes for at least two decades now. In an emergency situation, the driver will apply the brakes hard and feel the pedal pulsate under the foot as the brake releases and reapplies pressure several times a second.

Antilock brake systems, or ABS, will stop the vehicle faster and help the driver keep control.

Newer heavy-duty trucks and trailers are also equipped with ABS systems. But remember, they’re air brakes. Because of the heavy braking system’s massive size and the amount of air line involved, the brakes just pulsate at a much lower rate.