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The yellow, four-sided button on the dash controls the parking brakes on the truck or bus.

Applies and releases the truck or bus' parking brakes and is found on the vehicle's dash.

The trailer supply valve turns air off and on to the semi-trailer and subsequently controls its parking brakes.The valve is an octagon-shaped red button on the dashboard.

On most trucks, the air lines to the trailer are plumbed into the tractor protection valve.The tractor protection valve protect the tractors air supply from being depleted in the event of a trailer breakaway.

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.

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.

As part of the air brake system, all tanks are fitted with a safety valve to evacuate excess pressure in the event of a system failure.All tanks have a safety valve.

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.Several relay valve throughout the air brake system reduce brake lag, thus allowing the brakes to apply faster.

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 event of a trailer break away, the relay emergency valve is required to apply the trailer service brakes for a minimum of 15 minutes.

After a brake application, air is exhausted from the air brake system via the quick release valve to reduce brake lag.The quick release valve moves to reduce brake lag in the air brake system.

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Automatic Front Wheel Limiting Valve

Category: CDL Air Brake Valves 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…
A bobtail proportioning valve will move most of the braking to the front steer axles when there's not trailer attached.

Bobtail Proportioning Valve

Category: CDL Air Brake Valves Author: Rick
A bobtail proportioning valve great increases braking safety owing to different weight distribution.
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

Category: CDL Air Brake Valves 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…
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

Category: CDL Air Brake Valves 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…
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

Category: CDL Air Brake 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…
After a brake application, air is exhausted from the air brake system via the quick release valve to reduce brake lag.

Quick Release Valve

Category: CDL Air Brake Valves Author: Rick
The quick release valve moves to reduce brake lag in the air brake system.
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Relay Emergency Valve

Category: CDL Air Brake Valves 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…
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

Category: CDL Air Brake Valves Author: Rick
Several relay valve throughout the air brake system reduce brake lag, thus allowing the brakes to apply faster.
As part of the air brake system, all tanks are fitted with a safety valve to evacuate excess pressure in the event of a system failure.

Safety Valve

Category: CDL Air Brake Valves Author: Rick
All tanks have a safety valve.
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Spring Brake Modulator (Inversion) Valve

Category: CDL Air Brake Valves 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.
On most trucks, the air lines to the trailer are plumbed into the tractor protection valve.

Tractor Protection Valve

Category: CDL Air Brake Valves Author: Rick
The tractor protection valve protect the tractors air supply from being depleted in the event of a trailer breakaway.
The trailer supply valve turns air off and on to the semi-trailer and subsequently controls its parking brakes.

Trailer Air Supply Valve

Category: CDL Air Brake Valves Author: Rick
The valve is an octagon-shaped red button on the dashboard.
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Two-way Check Valves

Category: CDL Air Brake 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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