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How Do Diesel Hydraulic Brakes Work?

Apr 20, 2021 ·

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The braking system is one of the most important safety features on a vehicle. For diesel vehicles in particular, it’s important that a technician knows how the specific brake system functions so it can be maintained and serviced as needed.

Depending on the size of the diesel vehicle, either hydraulic brakes or air brakes are used. Air brakes are typically used when there is a large amount of weight or goods being transported, since hydraulic fluid has the potential to develop leaks.

Wondering about the specifics of when hydraulic brakes are used and how they work? Keep reading to find out more about them.

What Are Hydraulic Brakes?

Hydraulic brakes operate in the same manner as standard automotive brakes and are used as the main braking system on almost all passenger vehicles. Hydraulic brakes use fluid to transmit force when the brakes are applied.

A diesel vehicle’s hydraulic braking system operates on the principle that it’s impossible to compress fluid. All hydraulic brakes, from large diesel trucks to bicycles, rely on this principle. You can transmit force from a braking mechanism to a vehicle’s wheel-mounted brake unit simply by connecting the two components with a liquid-filled hose.

Hydraulic braking systems incorporate boosters, special brake lines and force multipliers to provide the adequate leverage needed to slow down and stop multiple-ton vehicles. However, the guiding principle remains the same: You can’t compress a liquid.

Very large diesel trucks (over Class 7) use air brakes, which are more efficient at slowing and stopping extremely heavy loads. Some smaller diesel trucks use hydraulic brakes, which operate on the same principles that automotive style brakes do.

Let’s review how both hydraulic and air brakes work, and why a manufacturer would choose one system over the other.

Hydraulic Brakes vs. Air Brakes

As stated above, hydraulic brake fluid does not compress. The force applied at the braking mechanism translates into force at the wheel assembly. The brake booster multiplies pressure, so a slight depression of the brake pedal can slow and stop a vehicle that weighs thousands of pounds.

Hydraulic brake system parts include:

  • Brake pedal
  • Brake pedal linkage/assembly
  • Brake booster
  • Master cylinder/fluid reservoir
  • Brakes lines
  • Proportioning valve
  • Wheel assembly (caliper rotor and pads, or brake drum and brake shoe)

Air brakes, on the other hand, operate on the principle of stored, compressed air. Air pressure acts upon the wheel brake assembly to operate the brakes and safely slow the vehicle. Air brakes are common on large semitrailers because they can couple and uncouple very easily and handle very heavy loads.

Air brake system parts include:

  • Air compressor
  • Air tanks
  • Governor/pressure regulator
  • Brake pedal
  • Wheel assembly (brake drums/shoes, S-cam)
  • Spring brakes (backup, safety and parking brakes)

Advantages of Hydraulic Brakes

Compared with their air brake counterparts, hydraulic braking systems offer a few advantages. The primary advantage for hydraulic brakes is their cost, both initially and for maintenance.

Other advantages include:

  • Easier to operate smoothly
  • No need for air dryers
  • Operate well in all temperatures

Disadvantages of Hydraulic Brakes

  • Do not tolerate minor system leaks as well as air brakes
  • Difficult to couple and uncouple trailers
  • Require more maintenance than air brakes

Special tools and procedures exist for removing all of the air from hydraulic brake systems, which is also known as “bleeding the system.”

As stated earlier, hydraulic fluid will not compress, but air will. Without proper bleeding of the system, hydraulic brakes will be less efficient and the foot pedal can feel soft or spongy.

Hydraulic Truck Brakes, Drum vs. Disc

Modern heavy-duty diesel trucks predominantly use drum brakes despite the widespread use of disc brake technology in the automotive world.

Diesel students work on a set of brakes in a UTI lab.

This is because the extreme heat associated with heavy-duty truck braking applications can overwhelm brake discs. Cracked and warped disc brake rotors were major problems in early attempts to transition to disc brakes in the diesel industry.

With advancements in technology and the ability to better dissipate heat, some diesel truck manufacturers are starting to use disc brakes on the front/steering axles. Time will tell if disc brakes become the norm on drive/trailer axles.

Identifying Potential Brake Problems

Like any other part of a vehicle, the hydraulic brake system may encounter problems on the road. Having an understanding of what can cause some of these issues can be helpful when talking to a technician or performing maintenance of your own:


Learn More About Hydraulic Brakes at UTI

Interested in finding out more about how the ins and outs of hydraulic brake systems, along with the other braking systems diesel vehicles use?

UTI’s Diesel Technology program has an entire course covering Diesel Truck Brake Systems & Chassis. Students learn all about hydraulic brakes and air brakes, from component identification to troubleshooting failures.

Apart from learning about truck braking systems and chassis, the program is designed to provide you with the knowledge and skills needed to succeed in an exciting role a diesel technician.1 And the program takes just 45 weeks to complete.7 To learn more, visit our program page and request more information.

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