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The drum or

There are 4 types of foundation brakes you must know for your CDL test.

Disc Brakes | Wedge Brakes | Air-Over-Hydraulic Brakes

Updated Jan 2021 

S-Cam Brakes OR Drum Brakes

This type of foundation brake can also be called "The Drum Brake."

The brake assembly consists of the braking parts around the wheel that are operated by the air brake system.

  • Brake chambers
  • Push rods
  • Slack adjusters
  • S-cam
  • Brake linings
  • Brakes drums.

These are the most common type of foundation air brakes that are currently found on commercial trucks.

And owing to advances in air brake technology and regulation in the last two decades, the only susceptibility of these brakes is brake fade.

Owing to air brake courses that are a requirement of any commercial licence operating an air brake equipped vehicle, this failing is almost non-existent.

Disc Brakes

Although uncommon up to this juncture in air brake technology, disc brakes are beginning to make inroads into the commercial industry.

These foundation brakes have been used on commercial vehicles in Europe for a number of years.

The reason for disc brakes technology is that these brakes do not experience brake fade.

Rather, when the rotor is heated, it expands into the brake pads and the brakes become more aggressive.

Think of your thumb and forefinger pinching the spine of a book to pick it up.

Disc brakes are the same principle.

The brake pads pinch the rotor (circular steel plate), creating friction which slows the vehicle.

Up to this juncture in automotive engineering these types of brakes have been popular on sports cars and bikes.

And these brakes are widely used on motorcycles and race cars because these actually work better when heated due to the rotor expanding into the brake pads.

Disc brakes have rotors, callipers, and brake pads. These types of brakes are beginning to appear on large vehicles.

The challenge for engineers up to this point is the sheer amount of heat generated by brakes on commercial vehicles.

Too much heat causes the component parts of the disc brakes to weld together and fail.

There are some newer trucks that have disc brakes.

These brakes cannot be adjusted or measured for adjustment by a driver.

These brakes must be serviced by a qualified person.

At one and the same time however, that the components of an air brake disc system are visually inspected daily by drivers for damage, insecure parts or components that are not working.

Knowledge Test Questions: which types of brakes have rotors?

Which types of brakes have callipers?

Can disc brakes be adjusted by the driver?

Disc brakes consist of:

  • brake pads;
  • callipers;
  • rotors.

Wedge Brakes


This type of brakes uses one or two small air chambers with wedge-shaped pushrods.

Most wedge brakes have internal automatic adjusters.
Knowledge test Question: how many brake chambers can a wedge brake have:

Answer: 1 or 2.

Wedge brakes must be checked and adjusted by a qualified mechanic.

The driver can look for damaged and insecure components.

The tug tests will also reveal if the brakes are working properly.

Wedge brakes were unreliable and couldn't be adjusted by the driver. Due to these limitations, they fell out of favours as the braking system on large vehicles.

Again, these types of brakes are rare.

These can be identified because the the brake chamberand push-rod are facing the wheel assembly.

Air-Over_Hydraulic Brakes


Although uncommon in air brake technology, air-over-hydraulic brakes are found every now and again.

These systems were most prevalent on 5-ton straight trucks equipped with "gas" engines.

If a truck or bus is equipped with air-over-hydraulic brakes, the driver must have an air brake endorsement to operate this vehicle.

Exception: If the vehicle is NOT equipped with an air treadle (a foot valve) to operate the service brakes, the driver does not require an air brake endorsement.

These systems neither worked well nor continued with wide-spread adoption of diesel engines.

In the ole' days of trucking, gas engines created a vacuum, and this air vacuum powered the air portion of the air-over-hydraulic braking systems.

Diesel engines on the other hand, do not create a vacuum and when most trucks were fitted with diesel engines in the 1970s and 80s, air-over-hydraulic brakes were no longer feasible.

To operate an air brake system, diesel motors had to be fitted with an air compressor, and once fitted, a pure air brake system simply made sense.

The adoption of diesel engines made air-over-hydraulic brakes obsolete. An air brake compressor was added making it an air brake system.

The system essentially takes a brake chamber (pressure intensifier) and fits it to a hydraulic master cylinder.

With the use of air pressure, the aim was to increase braking power in a hydraulic braking system.

These brakes cannot be adjusted or measured for adjustment by a driver.

These brakes must be serviced by a qualified person.

At one and the same time however, that the components of an air air-over-hydraulic system are visually inspected daily by drivers for damaged, loose or components that are not working.

As well, the hydraulic components must be check for leaks and functionality.

S-Cam OR Drum Brakes | Wedge Brakes | Air-Over-Hydraulic Brakes

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