Service Brake Subsystem

1. Brake Pedal

Pressing the brake pedal, also known as the foot valve treadle, applies the air brakes similarly to how stepping on the brake pedal applies the brakes in a car. The treadle of a foot valve has a springy feel, which is quite different from the hydraulic brake pedal of a car. You don’t need to press harder on a foot valve to apply more braking force; you simply press it down a bit farther. If the foot valve is held in one position, the air pressure delivered to the brake system remains constant.
Releasing the foot valve allows the application air to be exhausted through the assembly’s exhaust ports into the atmosphere. Essentially, it acts as a foot-controlled pressure regulator, enabling you to select any application pressure needed for either a gentle or a rapid stop.
A unique feature of a foot control valve is its ability to maintain the chosen application pressure even if there are small leaks downstream from the foot valve. By maintaining the treadle position, the foot valve will automatically open momentarily to replenish any lost air and then close again, ensuring consistent braking pressure.

2. How Air Brakes Work?

Brakes Applied

In this simplified diagram, air at full system pressure is indicated by the dark shading in the line connecting the supply reservoir to the foot valve. The driver is making a brake application. This can be seen by the light shading in the air lines connecting the foot valve to the air chambers. Arrows show the direction of air flow. The air chambers are pressurized and the brake linings have contacted the brake drums, slowing the vehicle.

Brakes Released

In this simplified diagram, the driver’s foot is off the brake pedal, allowing the brakes to release. This action has caused an exhaust port in the bottom of the foot valve to open, allowing the air that was applied to the brake chambers to escape. Note the burst of exhaust air below the foot valve. The return springs in the air chambers have returned the pushrod assembly to the released position, and the slack adjusters and s-cams have rotated to their released position. Brake shoe return springs (not shown) have retracted the brake linings away from the brake drums.

3. Dual air brake systems

The device that made dual systems possible is the dual foot valve. It’s actually two control valves operated by a single pedal. This allows the brake system to be divided into two completely independent sections. Each section has its own supply, delivery and exhaust ports.

The two sections of the dual foot valve are the primary and secondary. The primary section is located closest to the pedal, and in many systems operates the drive axle brakes. The secondary usually operates the steering axle brakes.

When the driver applies the brakes, both sections of the dual foot valve are activated. Air from the primary tank is applied to the rear axle brakes and air from the secondary tank is applied to the front axle brakes.

Most dual systems use three reservoirs: a supply reservoir and two service reservoirs, one for each section of the dual system. Each service reservoir is filled through a one-way check valve, and there are two reservoir pressure gauges, one for each service reservoir.

Even if one or the other system totally fails, the driver is able to make a controlled stop using only the foot valve, although maximum braking power will be reduced.

There are other ways of splitting a dual air brake system. However it’s divided, if one of the systems fail, the driver is still able to make a controlled stop.

Note the change in terminology for the reservoirs. The first reservoir (wet tank) is called the supply reservoir. The two service reservoirs are called the primary reservoir and secondary reservoir, indicating the section of the dual foot valve that they supply.

Some dual systems have the low-air warning device connected to the supply reservoir as shown, while others have two separate connections, one located on each service reservoir.

4. Air Loss Rate in Air Brake System

The air loss rate in an air brake system is a critical measure of the system’s integrity and performance. It refers to the amount of air pressure lost over a period of time when the brakes are not applied. Monitoring the air loss rate helps in identifying potential leaks or faults within the system that could compromise braking efficiency and safety.

A properly functioning air brake system should maintain a minimal air loss rate. Regular checks are necessary to ensure that the air loss rate stays within acceptable limits. These checks typically involve observing the pressure gauges and noting any drops in pressure over a specified period when the vehicle’s engine is off and the brakes are not applied.

Maximum Allowable Air Loss Rates

To ensure safety, there are established maximum allowable rates of air loss for different types of vehicles equipped with air brake systems. These rates are specified to help drivers and maintenance personnel determine when a vehicle may need servicing to address potential leaks or other issues within the air brake system.

Straight Truck:

• The maximum allowable air loss rate for a straight truck is 3 psi (pounds per square inch) per minute. This means that during a test, if the air pressure drops more than 3 psi within one minute, the system may have a leak or another issue that needs to be addressed.

Tractor and Trailer:

• For a combination of a tractor and a single trailer, the maximum allowable air loss rate is 4 psi per minute. This slightly higher rate accounts for the additional connections and components in the combined vehicle setup, which can present more opportunities for air loss.

Tractor with Two or More Trailers:

• In the case of a tractor hauling two or more trailers, the maximum allowable air loss rate is 6 psi per minute. The increased complexity and length of the air brake system in such configurations can lead to a higher potential for air loss, hence the higher allowable rate.

Importance of Monitoring Air Loss Rate

Maintaining the air loss rate within these specified limits is crucial for the safe operation of vehicles equipped with air brakes. Excessive air loss can lead to insufficient braking power, increasing the risk of collisions. Regular inspections and maintenance are essential to ensure that the air brake system is functioning correctly and to address any issues before they become critical.

5. Front Wheel-Limiting Valve

The front wheel-limiting valve is a crucial component in air brake systems, designed to control the amount of air pressure applied to the front brakes. This valve is essential for managing the braking force, especially under varying load conditions and road surfaces. Its primary function is to prevent the front wheels from locking up during braking, which can lead to a loss of steering control and increase the risk of skidding, particularly on slippery surfaces.

Types of Front Wheel Limiting Systems

Manual Front Wheel-Limiting Systems:

Manual front wheel-limiting systems require the driver to adjust the valve settings based on the road conditions and vehicle load. Typically, these systems have a control knob or lever inside the cab that the driver can manipulate. The settings may include positions like “normal” and “slippery,” which correspond to different levels of air pressure reduction.

• Normal Setting: In this position, the valve allows the maximum air pressure to the front brakes, providing full braking force. This setting is used under typical driving conditions where the road surface is dry and traction is good.
• Slippery Setting: When set to this position, the valve reduces the air pressure to the front brakes, lowering the braking force. This setting is useful in conditions where the risk of wheel lock-up is higher, such as on icy, snowy, or wet roads. By reducing the pressure, the front wheels are less likely to lock up, allowing the driver to maintain better steering control.

Automatic Front Wheel-Limiting Systems:

Automatic front wheel-limiting systems adjust the air pressure to the front brakes without driver intervention. These systems use sensors and control modules to detect road conditions and adjust the brake pressure accordingly. This automation ensures optimal braking performance and safety without requiring the driver to manually adjust settings.

• Operation: The system continuously monitors factors such as wheel speed, traction, and vehicle load. Based on these inputs, it dynamically adjusts the air pressure to the front brakes to prevent lock-up.
• Advantages: Automatic systems enhance safety by providing consistent and immediate adjustments to changing conditions, reducing the risk of human error. They also improve convenience, as the driver does not need to make manual adjustments.

Key Benefits of Front Wheel-Limiting Valves:

• Enhanced Control: By preventing front wheel lock-up, these valves help maintain steering control during braking, especially on slippery surfaces.
• Improved Safety: Reducing the risk of skidding and loss of control contributes to overall vehicle safety.
• Adaptability: Both manual and automatic systems allow for adjustments based on driving conditions, ensuring optimal braking performance in various scenarios.

6. Quick Release Valve

The quick release valve is a vital component in an air brake system, designed to expedite the release of air pressure from the brake chambers, allowing the brakes to release more quickly. This valve is essential for ensuring that the brakes respond promptly and effectively, both when applied and when released. By quickly exhausting air from the brake chambers, the quick release valve helps improve the efficiency and safety of the vehicle’s braking system.

Function of the Quick Release Valve:

• Operation: When the brake pedal is released, the quick release valve opens to allow the air pressure in the brake chambers to be vented quickly into the atmosphere. This rapid release of air pressure ensures that the brake shoes or pads disengage from the brake drums or rotors without delay.
• Mechanism: Inside the quick release valve, a diaphragm or piston moves to open an exhaust port when the pressure from the control line drops. This port allows the air in the brake chambers to escape rapidly.

7. Relay Valve

Relay valves are essential for reducing brake lag time, and ensuring rapid application and release of the brakes. They allow for a high volume of air to be delivered quickly to the brake chambers, enhancing the responsiveness and effectiveness of the braking system.

Function of the Relay Valve:

• Installation: Relay valves are positioned between the air reservoir and the brake chambers. They are typically installed close to the air chambers they control, often mounted on the vehicle’s frame rail.
• Operation:When the driver applies the brake pedal, the foot valve sends a control signal to the relay valve. The relay valve then opens and allows air from the reservoir to flow directly into the brake chambers, reducing the time it takes for the brakes to engage.
• Mechanism:Inside the relay valve, a diaphragm or piston responds to the control signal from the foot valve. This mechanism ensures that the pressure of the air delivered to the brake chambers matches the pressure of the control signal from the foot valve. For example, if the foot valve sends a signal for 20 psi (138 kPa) of pressure, the relay valve will deliver approximately 20 psi of air pressure to the brake chambers.

8. Two-way Check valves

Two-way check valves are essential components in air brake systems, designed to ensure reliable and safe operation by managing air pressure from multiple sources. These valves allow air to flow from two different sources into a single line, automatically selecting the higher pressure source to maintain consistent and adequate air pressure within the system.
The two-way check valve has two inlet ports connected to different air sources and one outlet port connected to the air system. Inside the valve, a shuttle or ball moves to block the lower pressure port, allowing air from the higher pressure source to flow through to the outlet port. This mechanism ensures that the air brake system always operates with the highest available pressure, enhancing reliability and performance. Additionally, the valve prevents backflow, stopping air from flowing back into the lower pressure source, which helps avoid contamination and pressure loss. Commonly used in dual air systems, two-way check valves manage air from primary and secondary reservoirs, providing redundancy and enhancing safety. 

9. Brake Hoses and Tubes

Brake hoses and tubes are essential components in an air brake system. They serve the critical function of directing compressed air to various parts of the braking system, ensuring that the brakes operate efficiently and effectively. These components form the network through which air travels from the compressor to the brake chambers, facilitating the application and release of the brakes.

It is crucial to ensure that brake hoses and tubes are intact and free from leaks to maintain effective brake operation. Any damage or leaks in these components can lead to a loss of air pressure, resulting in reduced braking efficiency or even brake failure. Regular inspections and maintenance are vital to identify and address potential issues before they become serious safety hazards.

Types of Brake Hoses and Tubes:

Brake Hoses:

• Flexible: Made of durable, flexible materials such as reinforced rubber or synthetic compounds. These hoses can bend and flex, making them ideal for connections between moving parts, such as the chassis and axle.
• Connections:Typically used to connect rigid brake lines to the brake chambers or other components where movement occurs.

Brake Tubes:

• Rigid: Made of materials such as steel or copper-nickel alloy, which provide strength and resistance to pressure. These tubes are used for longer, straight runs where flexibility is not required.
• Connections:Often used to run along the frame of the vehicle, providing a stable path for air to travel from the compressor to various components.

Common Issues and Solutions:

Leaks:

• Identification: Leaks can often be identified by listening for hissing sounds or using soapy water to detect bubbles at connections and along the hoses and tubes.
• Solution: Tighten loose connections or replace damaged sections to stop leaks.

Wear and Abrasion:

• Identification: Look for signs of physical wear, such as worn spots where hoses may be rubbing against other parts of the vehicle.
• Solution: Reroute hoses or use protective sleeves to prevent further abrasion.

Corrosion:

• Identification: Corrosion is common in metal brake tubes and can be identified by rust or pitting on the surface.
• Solution: Replace corroded sections with new tubes made from corrosion-resistant materials.

10. Air Brake Chambers

Air brake chambers are crucial components in an air brake system. They convert compressed air into mechanical force, which is used to apply the vehicle’s brakes. These chambers are typically mounted on the axles and connected to the brake system, where they receive compressed air from the brake lines when the driver applies the brakes. The force generated by the air brake chambers is transferred to the brake shoes or pads, pressing them against the brake drums or rotors to slow down or stop the vehicle.

The pushrod stroke within the air brake chamber is crucial for effective braking. The pushrod moves when compressed air enters the chamber, converting air pressure into mechanical force. The length of the pushrod stroke directly affects the braking force applied. Proper adjustment and maintenance of the pushrod stroke are essential to ensure the brakes operate efficiently and safely.

Function of Air Brake Chambers:

• Operation: When the driver applies the brakes, compressed air from the brake lines enters the air brake chamber through the supply port. This air pressure pushes against a diaphragm inside the chamber.
• Mechanical Force: The diaphragm is connected to a pushrod, which extends as the diaphragm moves. The movement of the pushrod converts the air pressure into mechanical force, which is then transferred to the brake shoes or pads.
• Braking Action: This mechanical force presses the brake shoes or pads against the brake drums or rotors, creating friction that slows down or stops the vehicle.

Components of an Air Brake Chamber:

Diaphragm:

• Function: Separates the air and mechanical sections of the chamber. It moves in response to air pressure, pushing the pushrod outward.

Pushrod:

• Function: Converts the movement of the diaphragm into mechanical force that applies the brakes. The pushrod’s stroke length is critical for effective braking.

Return Spring:

• Function: Helps return the diaphragm and pushrod to their original positions when the air pressure is released, ensuring the brakes disengage properly.

Mounting Brackets:

• Function: Secure the air brake chamber to the vehicle’s axle, ensuring stable and reliable operation.

Air Supply Ports:

• Function: Allow compressed air to enter and exit the chamber, controlling the movement of the diaphragm and pushrod.

11. Air Brake Chamber Styles, Types, Sizes, and Pushrod Stroke Adjustment Limits

Air brake chambers come in various styles, types, and sizes, each designed to meet specific requirements of different vehicles and braking systems. Understanding these variations and the regulated pushrod stroke adjustment limits is crucial for ensuring proper brake performance and safety. This slide will help you identify the different styles, types, sizes, and the importance of maintaining the correct pushrod stroke adjustment limits.

Styles of Air Brake Chambers:

Service Chambers:

• Function: Used primarily for service braking applications.
• Components: Typically include a diaphragm, pushrod, and return spring.
• Application: Commonly found on the front axles of trucks and buses.

Spring Brake Chambers:

• Function: Combine service braking and parking/emergency braking functions.
• Components: Include a powerful spring mechanism that applies the brakes when air pressure is lost.
• Application: Usually installed on the rear axles of trucks and trailers.

Types of Air Brake Chambers:

Type 30:

• Description: A standard size for heavy-duty applications.
• Pushrod Stroke Limit: Typically around 2.5 inches (6.35 cm).
• Application: Commonly used in large trucks and trailers.

Type 24:

• Description:Slightly smaller than Type 30.
• Pushrod Stroke Limit: Typically around 2 inches (5.08 cm).
• Application: Suitable for medium-duty vehicles.

Type 20:

• Description:Smaller chamber for lighter applications.
• Pushrod Stroke Limit: Typically around 1.75 inches (4.45 cm).
• Application: Typically around 1.75 inches (4.45 cm).

Sizes of Air Brake Chambers:

• Determination: The size of an air brake chamber is often indicated by a number, such as Type 30, Type 24, or Type 20, which corresponds to the effective area of the diaphragm in square inches.
• Importance: The size determines the force the chamber can generate; larger chambers produce greater force, suitable for heavier vehicles.

Regulated Pushrod Stroke Adjustment Limits:

• Definition: The pushrod stroke is the distance the pushrod travels when air pressure is applied to the brake chamber.
• Importance: Correct adjustment of the pushrod stroke is essential to ensure the brakes apply the appropriate force. If the stroke is too long or too short, it can lead to inadequate braking or excessive wear on brake components.

This table includes both the adjustment limits and the maximum stroke values for each type of brake chamber, ensuring that the information is accurate and comprehensive for proper brake maintenance and safety checks.

Brake Adjustment Limits for Clamp Type Brake Chambers

12. Brake Chamber Video

13. Brake Adjusters (Slack Adjusters)

Brake adjusters, also known as slack adjusters, are critical components in an air brake system. They are used to maintain the correct amount of slack in the brake linkage, ensuring that the brakes engage properly and consistently. Slack adjusters convert the linear motion of the pushrod into rotational motion needed to apply the brakes, and they automatically adjust for wear on the brake shoes or pads to maintain optimal brake performance.

Types: There are two main types of slack adjusters:

• Manual Slack Adjusters: Require periodic adjustment by a mechanic to maintain the correct brake shoe clearance.
• Automatic Slack Adjusters: Automatically adjust to compensate for brake wear, reducing the need for manual adjustments and ensuring consistent brake performance.