What Are the Three Parts of an Air Brake System?

The three parts of a heavy-vehicle air brake system are the service brake system, the parking brake system, and the emergency brake system. These distinct but interconnected subsystems work together to provide controlled deceleration, secure holding when parked, and automatic braking if air pressure is lost.

The Three Parts Explained

At the heart of modern truck and bus braking, these three systems divide responsibilities: routine braking, secure parking, and fail-safe response. Understanding their roles helps drivers operate safely and identify problems early.

• Service brake system: The primary, foot-operated brakes used to slow or stop the vehicle during normal driving.
• Parking brake system: Spring-actuated brakes that hold the vehicle stationary when parked, engaged by the parking control valve.
• Emergency brake system: A safety backup that applies the brakes automatically (typically via spring brakes) if air pressure falls to unsafe levels or a major failure occurs.

Together, these systems ensure a vehicle can brake reliably in everyday conditions, remain secured when stationary, and protect occupants and road users during a loss-of-air event.

How Each System Works

Service Brake System

The service brakes are controlled by the foot pedal. When the driver applies the brake, air flows through control valves to brake chambers at the wheels, converting air pressure into mechanical force to slow the vehicle. Anti-lock braking systems (ABS) modulate pressure to prevent wheel lockup and help maintain steering control.

Key elements and actions that define the service brake system include the following components and processes.

• Driver input via the foot (treadle) valve regulates air pressure sent to the wheel ends.
• Relay and quick-release valves speed up application and release, especially on long wheelbase vehicles and trailers.
• Brake chambers and slack adjusters (with S-cams or discs) convert air pressure to braking torque.
• Dual-circuit design (front/rear or primary/secondary) provides redundancy if one circuit fails.
• ABS wheel speed sensors and modulators reduce lockup risk and improve control under hard braking.

Because the service system is used constantly, proper adjustment, air supply integrity, and ABS functionality are critical for predictable stopping performance.

Parking Brake System

Parking brakes use powerful mechanical springs in spring brake chambers to hold the vehicle when parked. Air pressure keeps these springs compressed during driving; releasing air (by pulling the parking control) lets the springs expand and apply the brakes.

Typical features of the parking brake system are listed below to clarify how it secures a vehicle at rest.

• Spring brake chambers provide mechanical holding force independent of service air pressure.
• The parking control valve (often the yellow diamond knob) vents air to set the brakes.
• Air pressure must be adequate before releasing parking brakes; low pressure may prevent release.
• On tractors and trailers, separate controls and protection valves manage each unit’s parking brakes.

Because parking brakes rely on mechanical springs, they remain applied even if air leaks occur, ensuring the vehicle stays secured when parked.

Emergency Brake System

The emergency brake system acts automatically if air pressure drops dangerously low or a major failure occurs. In most designs, the same spring brakes used for parking apply to stop or hold the vehicle when supply pressure collapses.

The following points outline how the emergency system protects against loss-of-air incidents.

• Low-air warning typically activates around 60 psi (or above), alerting the driver to take action.
• Automatic spring brake application occurs as pressure falls, usually between 20–45 psi, to stop/secure the vehicle.
• Dual service circuits provide partial braking if one circuit fails, improving controllability during a fault.
• Trailer protection and breakaway features prevent uncontrolled trailer rollaway in a separation or rupture.

This fail-safe behavior is central to air brake safety, giving drivers a margin of protection when leaks, ruptures, or compressor issues arise.

Common Supporting Components

While not one of the three “parts,” several components enable the service, parking, and emergency systems to function reliably across heavy-duty operations.

• Air compressor and governor: Generate and regulate system air pressure.
• Air reservoirs (primary, secondary, and supply tanks): Store compressed air; include drain valves.
• Air dryer and filters: Remove moisture and contaminants to reduce corrosion and freezing.
• Check valves and pressure-protection valves: Prevent backflow and isolate circuits to maintain pressure.
• Foot (treadle) valve, relay valves, and quick-release valves: Control and speed brake applications.
• Brake chambers, slack adjusters, S-cams or disc calipers: Convert air pressure to braking force.
• ABS ECU, modulators, and wheel speed sensors: Enhance control under hard braking.
• Hoses and lines: Carry air between components; must be intact, properly routed, and leak-free.

Keeping these elements maintained—especially dryers, reservoirs, and valves—helps all three brake systems perform consistently in varied climates and loads.

Safety, Inspection, and Operation Tips

Routine checks and good driving practices preserve braking performance and ensure that the service, parking, and emergency systems function as designed.

• Drain moisture from reservoirs as required and ensure the air dryer is serviced on schedule.
• Perform daily air-brake checks: build pressure, check governor cut-in/cut-out, verify low-air warning (≈60 psi or above), and confirm spring brake automatic application (20–45 psi).
• Inspect slack adjusters, hoses, and chambers; address air leaks immediately—do not operate with audible or measurable leaks beyond limits.
• On long downgrades, use proper speed management and engine/exhaust braking; avoid overheating service brakes.
• Never “cage” spring brakes except for controlled service procedures; ensure parking brakes are fully set when leaving the cab.
• Confirm ABS warning lights extinguish after start-up; investigate persistent warnings before operation.

These practices help prevent brake fade, preserve stopping distances, and provide an essential safety margin in emergencies.

Bottom Line

The air brake system comprises three parts: the service brake system for normal stopping, the parking brake system for secure holding, and the emergency brake system for automatic protection during air loss. Understanding how each works—and maintaining the supporting air supply and control components—keeps heavy vehicles safe and compliant on the road.

What are three air braking systems?

Air brakes are really three different braking systems: service brake, parking brake, and emergency brake.
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What are three parts of an air brake system?

Three key parts of an air brake system are the air compressor, which generates compressed air; the reservoirs (or tanks), which store it; and the control valves and brake chambers, which use the compressed air to activate the mechanical linkages, such as the brake shoes and drums, that create the friction needed to slow or stop the vehicle.
 
Here’s a breakdown of their functions:

1. Air Compressor: This component pumps air into the system, pressurizing it for use by the brakes. 
2. Reservoirs (Air Tanks): These tanks hold the compressed air, providing a reserve that can be used when the driver applies the brakes. 
3. Control Valves & Brake Chambers: 
   • Control Valve (Brake Pedal): This valve, operated by the driver, regulates the flow of air from the reservoirs to the brake chambers. 
   • Brake Chambers: These are activated by the air pressure from the valve. 
   • Foundation Brakes (S-cam, Slack Adjusters, Brake Shoes & Drums): The brake chambers push a rod, which moves the slack adjuster and S-cam. The S-cam forces the brake shoes against the inside of the brake drums, creating friction to stop the vehicle. 

Together, these parts form a system where compressed air provides the power to apply mechanical forces and bring a vehicle to a stop.

What is the 3rd step of the air brake test?

The 3rd step of the standard three-step air brake test is the trailer and parking brake (or spring brake) valve pop-out test, where you continue to pump the brakes to drain air pressure until both the red (tractor protection) and yellow (trailer control) buttons pop out, indicating the valves closed correctly between 20 and 45 PSI. 
Here are the steps for the three-step air brake test:

1. Leak Test: Press the brake pedal and hold it to check for air leaks. After the initial pressure drop, monitor the gauges for one minute; the pressure should not drop more than 4 PSI. 
2. Low Air Warning Test: Continue fanning the service brake to lower the air pressure. The low air warning light and buzzer should activate at or before 55 PSI. 
3. Trailer and Parking Brake Pop-Out Test: Continue fanning the brakes. The tractor (red) and trailer (yellow) protection valve buttons should pop out when the air pressure falls between 20 and 45 PSI. 

What are the three main components of the brake system?

These are fixed calipers and floating calipers. Just like other brake parts, you need to brake calipers if they show signs of leaking the brake fluid. Brake pads, rotors, and calipers are the three main components of a brake disc system.