fbpx

Cart

The cart is empty

Cannot find your purchased courses? Click here to Login!

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

OBSOLETE

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

OBSOLETE

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

Log in to comment
blog image

Brake Fade | Air Brakes

Author: Rick
Although air brakes are more-or-less buletproof these, like all braking systems, have one failing: BRAKE FADE.
Default Image

Two-way Check Valves

Author: Rick
Must be used whenever two sources of air are used to operate a single device or system. For CDL air brake test, the driver must know there are 2 in the system.
Default Image

Trailer Hand Control (Spike)

Author: Rick
OBSOLETE Independently applies and releases the trailer service brakes. Knowledge Test Question: This air brake control is not to be used for parking. Although most tractors are still equipped with hand valves, beyond a trailer service tug test, these air…
On most trucks, the air lines to the trailer are plumbed into the tractor protection valve.

Tractor Protection Valve

Author: Rick
The tractor protection valve protect the tractors air supply from being depleted in the event of a trailer breakaway.
Default Image

Stop Light Switch

Author: Rick
Activates the brake lights when a service brake application is made by the driver.
Default Image

Spring Parking Brake (Emergency)

Author: Rick
Applies the parking brakes of the air brake equipped vehicle indefinitely – if needed of course. The parking brakes are held in the “ON” position by the mechanical force of the springs; these spring parking brakes are held in the “OFF” position by system air…
Default Image

Spring Brake Modulator (Inversion) Valve

Author: Rick
OBSOLETE Used on some vehicles with dual air systems. This device provides a means of using the force of the spring brakes to assist in stopping a vehicle in an event of a failure in the rear (primary) air system.
Default Image

Slack Adjuster

Author: Rick
Slack adjusters have three purposes: 1) To convert the forward linear motion of the push rod into rotary motion of the camshaft. The slack adjuster works on exactly the same principle as a box end wrench on a nut. 2) To act as a lever, thus generating greater…
Default Image

Air Tanks (Reservoirs)

Author: Rick
Stores compressed air for braking and for other air operated accessories. The air tanks are the first fail-safe in the air brake system. In the unlikely event that the air compressor fell of the side of the engine, the air tanks (reservoirs) store enough air…
The relay valve takes a message from the brake pedal and draws air directly from the air tank to the brake chamber, thus reducing brake lag.

Relay Valve

Author: Rick
Several relay valve throughout the air brake system reduce brake lag, thus allowing the brakes to apply faster.
Default Image

Relay Emergency Valve

Author: Rick
OBSOLETE Fills the trailer air tanks (reservoir) and applies and releases the trailer service brakes. The relay emergency valve also applies the trailer emergency brakes in the event of a trailer breakaway or normal disconnect. Knowledge Test Question: In the…
After a brake application, air is exhausted from the air brake system via the quick release valve to reduce brake lag.

Quick Release Valve

Author: Rick
The quick release valve moves to reduce brake lag in the air brake system.
You must know what a manual front wheel limiting valve is for your CDL air brake test. These are no longer found on modern trucks.

Manual Front Wheel Limiting Valve

Author: Rick
OBSOLETE To reduce application pressure to the steering axle by 50 % compared to the drive axle brakes, when required by reduced road conditions (rain, snow or ice). The system consists of a dash switch with 2 positions: Dry Road: 100% front wheel braking; in…
Air Brakes Simply Explained is a comprehensive manual that guarantees that you will pass your air brake ticket first time.

Brake Pedal - Foot Valve OR Treadle Valve

Author: Rick
MUST KNOW The brake pedal applies the service brakes - truck, or truck and trailer brakes. In other words, the brake pedal brings the vehicle to a stop when it it depressed. SHOULD KNOW The foot valve operates the secondary and primary sub systems…
Default Image

Compounding Air Brakes

Author: Rick
OBSOLETE Making a heavy service brake application while the parking brakes are applied. This is often an error of truck driving school students.
A caging bolt is used to release the emergency brakes in the event of an emergency (life or death situation).

Caging Bolt

Author: Rick
Allows the spring brakes to be manually released in an emergency - a dire emergency.
Default Image

Brake Linings & Drums

Author: Rick
To stop the vehicle, brake linings rub against the brake drum. This action creates friction, converting the vehicle's forward energy of motion into heat energy, which stops the vehicle’s tires. If the tires have traction, the vehicle will come to a stop. The…
Brake fade is the only reason that a modern air brake system will fail.

Brake Fade

Author: Rick
Brake faide is the only reason that a modern air brake system will fail.
A bobtail proportioning valve will move most of the braking to the front steer axles when there's not trailer attached.

Bobtail Proportioning Valve

Author: Rick
A bobtail proportioning valve great increases braking safety owing to different weight distribution.
Default Image

Automatic Front Wheel Limiting Valve

Author: Rick
To reduce application pressure to the steering axle brakes during most braking situations. Reduced pressure to the steer axle brakes occurs for most braking applications. Only during hard braking is there equivalent pressure delivered to both the front and…
Default Image

Air Gauge(s)

Author: Rick
The gauges are in either pounds per square inch (psi) or Kilopascals (kPa). These measure the amount of air pressure in the primary and secondary air tanks.
Default Image

Air Brake Chambers

Author: Rick
Convert air pressure into a strong mechanical force to apply the foundation brakes.
The one-way check valve is primary responsible for the division of the air brake system into a primary and secondary sub-system.

One-Way Check Valves

Author: Rick
A cattle chute with one-way gates allows the cows to only move in one direction – forward. A one-way check valve in an air brake system works exactly the same – it allows air to only move in one direction – forward through the system. The one-way check valves…
air brake adjustment

Air Brake Manual Adjustment – Pry Bar Method

Author: Rick
Using the pry bar method, confirm that the brake is in fact out of adjustment. After confirming that the brake is out-of-adjustment and it is equipped with a manual slack adjuster, complete the following steps to bring the brake back into adjustment. If one…
Default Image

Compressor or Build-up Test (Air Brakes)

Author: Rick
Depending on which province you're in, the air buildup time or compressor test has somewhat different paramenters. Still, all require that the diesel motor be at a fast idle. A fast idle on a diesel engine is between 900 &1200 rpm.
Default Image

Pry Bar - Check Air Brake Adjustment

Author: Rick
The National Safety Code madates that every driver of a commercial vehicle equipped with air brakes must check push rod travel every day. In many jurisdications, drivers are allowed to use the pry bar method; in others , they must use the applied stroke…
Default Image

Applied Stroke - Check Air Brake Adjustment

Author: Rick
The National Safety Code madates that every driver of a commercial vehicle equipped with air brakes must check push rod travel every day as part of his/her pre-trip inspection. In some jurisdications, drivers must use the applied stroke method; in others, they…

© Smart Drive Test Inc. 2024.

Contact Us: [email protected]