How Do Diesel Hydraulic Brakes Work?


Diesel Hydraulic Brakes Explained & Why They Are Important

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 truck’s classification, 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 diesel hydraulic brakes are used and how they work? Keep reading to find out more about them.

What Is a Hydraulic Brake System?

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.

How Do Hydraulic Brakes Work?

Hydraulic brakes work by transporting pressurized brake fluid directly to the wheel assembly to stop the vehicle. 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 Components

Various components work together to activate the hydraulic brake system. Let’s dive into each component’s role.

Brake Pedal

The brake pedal gives the driver access to the hydraulic brake system. When a driver hits the brake pedal, it places force on the push rod (a component of the brake pedal linkage/assembly).

Brake Pedal Linkage/Assembly

As mentioned above, the brake pedal linkage/assembly includes a push rod. When the push rod receives force from the pedal, it extends into the brake booster, where the operator’s pressure is multiplied. 

Brake Booster

Hydraulic brake boosters draw additional pressure from the power steering pump and increase it at the master cylinder to boost braking power.

Master Cylinder/Fluid Reservoir

The master cylinder contains pistons that create a hydraulic force and pressurize the entire hydraulic brake system. The fluid reservoir houses brake fluid to prevent air from entering the system.

Brakes Lines

The brake lines transport pressurized fluid from the master cylinder to the wheel assembly to stop the vehicle when activated by the pedal.

Proportioning Valve

The proportioning valve optimizes brake balance. It ensures the front and rear brakes receive the pressure necessary to safely stop the vehicle. 

Wheel Assembly

The wheel assembly includes the caliper rotor and pads, also known as the brake drum and brake shoe. The brake lines carry pressurized fluid to the caliper. Pistons inside the caliper then force the pads against the rotor, creating friction that stops wheel rotation.

Hydraulic Brakes vs. Air Brakes

As stated above, hydraulic brake fluid does not compress under force. Instead, it uniformly travels to the wheel assembly via brake lines to initiate friction between the rotor and pads and stop the vehicle.

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 diesel 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 diesel maintenance of your own:

Low Brake Pedal

  • Automatic adjusters
  • Linings / pads worn, contaminated or distorted
  • Drums too thin or cracked

Spongy Pedal

  • Drums too thin or cracked
  • Air in system / fluid contaminated

Hard Pedal

  • Linings / pads worn, contaminated or distorted
  • Backing plate lands dry, worn or distorted
  • Hoses / lines kinked or clogged
  • Wheel cylinder / caliper pistons seized
  • Defective master cylinder
  • Defective power booster

Pedal Fades

  • Drums too thin or cracked
  • Air in system / fluid contaminated
  • Wheel cylinder / caliper leaking
  • Hoses / lines leaking
  • Defective master cylinder

Noise (Click, Squeal, Scrape)

  • Wheel bearings loose
  • Automatic adjuster
  • Linings / pads worn, contaminated or distorted
  • Shoe return springs weak or broken
  • Drums too thin or cracked
  • Backing plate lands dry, rusted or grooved
  • Backing plate loose, worn or distorted
  • Caliper loose or misaligned


  • Wheel bearings loose
  • Alignment or uneven tire tread
  • Linings / pads worn, contaminated or distorted
  • Shoe return springs weak or broken
  • Drums too thin or cracked
  • Backing plate lands dry, rusted or grooved
  • Backing plate loose, worn or distorted
  • Caliper loose or misaligned
  • Wheel cylinder / caliper pistons seized
  • Tire pressure
  • Loose or worn suspension component

Chatter, Shudder, Pedal Pulse

  • Wheel bearings loose
  • Linings / pads worn, contaminated or distorted
  • Drums too thin or cracked
  • Backing plate loose, worn or distorted
  • Caliper loose or misaligned
  • Rotor has excessive runout or thickness variation
  • Drums out of round

Brakes Drag

  • Parking brake cables / linkage / adjustment
  • Automatic adjusters
  • Linings / pads worn, contaminated or distorted
  • Shoe return springs weak or broken
  • Backing plate lands dry, rusted or grooved
  • Backing plate loose, worn or distorted
  • Hoses / Lines kinked or clogged
  • Wheel cylinder / caliper pistons seized
  • Defective master cylinder

Hydraulic Brakes FAQs

Are hydraulic brakes more reliable than air brakes?

While rare in both brake systems, when it comes to failure air brakes are safer. When a leak depletes the fluid in a hydraulic brake system, the vehicle cannot stop. However, when air brake systems leak, the spring brake activates to bring the vehicle to a complete and controlled stop.

What maintenance is required for hydraulic brakes?

A trained diesel technician should service your hydraulic brakes once to twice a year. They will inspect and change brake fluid, if necessary, examine brake lines for corrosion, lubricate calipers and check brake pads.

How long do hydraulic brakes last?

The lifespan of hydraulic brakes can vary by driving conditions. As a general rule of thumb, replace brake pads every 25,000 to 65,000 miles and rotors every 30,000 to 70,000 miles.

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 for an exciting role a diesel technician.1 And the program can take just 45 weeks to complete.7 To learn more, visit our program page and request more information.

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