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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.

The air pressure stored in the air tanks is the maximum available pressure for a service brake application. In other words, if there is 80psi in the air tanks, the driver can only make an 80psi brake application.

This fact is a common test questions on air brake knowledge tests.

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Brake Fade | Air Brakes

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

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

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