Air Brakes vs Hydraulic Brakes? Which One Does Your Truck Use?

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Iron Buffalo Truck & Trailer
September 15, 2023
Air Brakes vs Hydraulic Brakes? Which One Does Your Truck Use?

Modern diesel trucks are available with two different options of braking systems. One system is  hydraulic brakes, whereas the other is air brakes. As an operator or fleet owner, you need to be aware of the best braking system required by your truck. This is a crucial topic since the answer directly influences vehicle safety, price, driver pool availability, and operational expenses.

To help guide the brake selection process, we’ll outline the different braking systems, including how they function, the proper vehicle size and application for each, and other considerations.

What are hydraulic brakes? 

Fluid is used to power hydraulic brakes. When the driver presses the brake pedal, the hydraulic fluid pressure rises to the point where the brake pistons at each wheel are forced to push the brake pad against the drum (or rotor in the case of disc brakes), causing friction, slowing the wheels, and eventually bringing the vehicle to a complete stop.

The technology for hydraulic brakes is quite similar to that used in passenger vehicles, and the components are substantially more significant in order to accommodate larger weight standards.

What are air brakes? 

As the name indicates, air brakes utilize air to provide stopping power rather than fluid. The brakes are deactivated when the air tanks are completely inflated. When the driver presses the brake pedal, air enters the braking chamber, prompting the chamber diaphragm, spinning the "S-cam," and finally pushing the brake pads on the brake drum the truck to a halt. The air is then released when the brake pedal is retracted, enabling the brakes to relax and the wheels to roll. The compressor restores the original air pressure in the system.

Air brakes vs. Hydraulic brakes

The unmistakable hiss of air coming off a large rig as it pulls to a halt next to you can be heard all over any major city and on roads nationwide. However, have you ever wondered why semi-trucks use air brakes? Why aren't they able to utilize hydraulic brakes like smaller vehicles?

It all boils down to resource availability and dependability. The more weight a vehicle has, the more probable it can deploy air brakes. Small automobile brake lines need hydraulic fluid to be supplied and maintained manually, while air is readily available and ready to be utilized in any truck braking system. But that's just one of the reasons they're so popular in the business.

Let's start with an explanation of how air brakes operate. A compressor fills storage tanks to a predetermined pressure, which a governor controls. A valve ensures that air only flows in one direction via the pipe, ensuring that the air tanks do not leak even if the compressor does. The air is then sent into the brake lines, where pressure variations (due to the brake pedal being pressed) are utilized to move a sequence of rods, cams, and brake components as required while driving.

Furthermore, if the brake line in the hydraulic system leaks, the whole system will fail since it will be unable to refill the pressure required to engage the brakes and slow the wheels. Hydraulics are just too risky to employ in a fully loaded truck or other heavy-duty equipment since their default setting does not slow the vehicle the same. They're also inconvenient: at a GVWR of around 26,000 lbs., the required equipment becomes too heavy, inefficient, and too hot to be usable during semi-truck operation.

Why do semi-trucks, on the other hand, utilize air brakes? 

The "inactive" option on these systems is closed, which means that if all of the pressure suddenly drops below 45 psi, the brakes will immediately engage. They use extremely powerful springs behind pistons that stretch out and keep all drive wheels in place until an air pressure of roughly 65 psi drives the pistons back into driving position. Semi-trucks and other tractor-type vehicles must have this emergency system for obvious reasons: they are safer overall, and their stopping strength is substantially larger. 

These emergency brakes are engaged by a simple switch on the dash, allowing the pressure to be released and the springs to take over, bringing the vehicle to a complete stop. Furthermore, a single truck may manage numerous trailers, allowing the driver to apply standard brakes for all trailers at the same time – and if the tractor splits from the others, all emergency parking brakes shut immediately.

If the lines are opened during maintenance on a hydraulic system, the whole system must be flushed out and the damaged line replaced to guarantee no air remaining (because the bubbles would react differently than the fluid inside, causing unexpected shifts in pressure). With air brakes, on the other hand, you need to repair a leaky or defective line, which saves you a lot of time and effort. Dual air brake systems, in which each axle has its own set of lines and storage tanks, may reduce the need to go through the whole system during maintenance.

Is there a disadvantage to employing air brakes? 

One is a buildup of water vapor in the pipes, which may create problems if it freezes during winter driving. This can be prevented with good maintenance and air dryers and drain valves. Because huge trucks' brakes don't transmit braking force as well, it may take a bit longer for them to slow down. Another factor to consider is that air brakes can only be used by drivers with a Class 1 license, requiring further training, knowledge, and financial compensation. But none of these drawbacks are significant compared to the tremendous increases in safety and dependability of air brake systems in semi-trucks and other commercial vehicles, as well as the fact that they are intended to leap into action even in the worst-case situation.