Air Brake System Diagram for Trucks, Buses and Trailers | VADEN ORIGINAL
Explore a labelled air brake system diagram for trucks, buses and trailers: compressor, dryer, protection valve, brake chambers and ABS/EBS by VADEN ORIGINAL.
This air brake system diagram maps the complete compressed-air braking circuit of a heavy commercial vehicle — truck, bus or trailer — from the moment air is generated to the instant each wheel is braked. Unlike a passenger car, these vehicles use compressed air rather than hydraulic fluid as the working medium, which is why every reservoir, valve and brake chamber has to be laid out clearly. The labelled illustration groups the system into four functional sections so truck technicians, fleet buyers and students can follow the airflow and understand how each component depends on the next.
Download the full Air Brake System Technical Guide (PDF)
Section one, Air Supply, is where compressed air is produced and conditioned: the engine-driven compressor feeds the air dryer, which removes moisture and oil before the air reaches the wet tank; from there the four-circuit protection valve divides the supply into independent circuits and seals off any circuit that loses pressure. Section two, Reservoir and Control, stores and meters that air — the primary and secondary tanks feed the two service circuits, the brake pedal (foot) valve delivers air on demand, the relay valve shortens response time at the rear axle, and the parking brake valve commands the spring brakes.
Section three, Brake Actuation, is where air pressure becomes mechanical braking force through the front, rear and spring brake chambers; the spring (combination) chambers also provide the parking and emergency brake. Section four, Electronics, overlays the pneumatic system with an ABS/EBS ECU and wheel-speed sensors that monitor each wheel and modulate pressure to prevent lock-up and shorten stopping distances. Read together, the four sections show how VADEN ORIGINAL braking components work as one coordinated safety system.
Related product categories: Air Brake Compressors · Air Dryer Valve · Relay Valve · Quick Release Valve · Brake Cylinder
Air brake system diagram: the air path in order
An air brake system diagram is easiest to read as a path rather than a picture. Air is drawn in through a filter, compressed by the engine-driven compressor, dried and cleaned in the air dryer, then stored in the reservoirs. From the reservoirs a multi-circuit protection valve divides the supply into separate circuits so that a leak in one cannot empty the others. Driver demand enters at the foot brake valve, is amplified locally by relay valves, and finally reaches the brake chambers where pressure becomes mechanical force. Spring brake chambers sit in the same path but work in reverse: they need air to stay released.
| Stage | Component | What happens here |
|---|---|---|
| 1 | Air filter and compressor | Ambient air is filtered and compressed by the engine |
| 2 | Governor | Charging stops at cut-out pressure and restarts at cut-in |
| 3 | Air dryer | Moisture and oil are removed; the cartridge is purged and regenerated |
| 4 | Wet and service reservoirs | Dry air is stored for the service and park circuits |
| 5 | Multi-circuit protection valve | Supply is split so one circuit failure cannot drain the rest |
| 6 | Foot brake valve | Driver demand is metered into the service circuits |
| 7 | Relay valve | Reservoir air is admitted locally to shorten response time |
| 8 | ABS modulator | Pressure is held, released or reapplied to prevent wheel lock |
| 9 | Service brake chamber | Air pressure becomes push-rod force at the wheel |
| 10 | Spring brake chamber | Spring applies the park brake; air is required to hold it off |
Dual-circuit layout: why the system is split
Heavy vehicles do not run a single brake circuit. The supply is divided so that the front axle, the rear axle, the park and trailer supply, and the auxiliary consumers each draw from a protected branch. If one branch develops a leak, the protection valve isolates it and the remaining circuits keep enough pressure to stop the vehicle. On a tractor and trailer combination the coupling adds two lines — supply and control — plus the electrical connection that carries ABS or EBS signalling. Reading any air brake diagram therefore starts with identifying which circuit each line belongs to, not with the individual valves.
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Frequently Asked Questions
- What does an air brake system diagram show?
- It shows the full compressed-air braking circuit of a heavy vehicle and how air flows from the compressor, through the air dryer, reservoirs and control valves, to the brake chambers at each wheel, plus the ABS/EBS electronics. In this diagram the components are grouped into four sections: air supply, reservoir and control, brake actuation, and electronics.
- How does a truck air brake system work?
- The compressor charges the reservoirs with compressed air through the air dryer, which keeps the system clean and dry. When the driver presses the brake pedal, the foot valve meters air to the front and rear brake chambers; inside each chamber a diaphragm pushes the pushrod and slack adjuster to apply the brake. Releasing the pedal exhausts the air and the brakes retract.
- What is the four-circuit protection valve for?
- It splits the air coming from the wet tank into separate, isolated circuits — typically primary service, secondary service, parking, and auxiliary or trailer supply. If one circuit develops a leak or fails, the valve protects the remaining circuits so the vehicle keeps its braking capability. This built-in redundancy is a core safety requirement for commercial vehicles.
- What is the difference between a service brake chamber and a spring brake chamber?
- A service (single-diaphragm) brake chamber applies the brake only when air pressure is supplied and is typically fitted on the steer/front axle. A spring (combination) brake chamber adds a powerful internal spring: air pressure holds the spring compressed to keep the brake released, and when pressure drops — for parking or in a failure — the spring applies the brake automatically. These are used on the drive/rear axle.
- What do ABS, EBS and ALB do in an air brake system?
- Wheel-speed sensors feed the ABS/EBS ECU, which modulates brake pressure to stop the wheels locking under hard braking, keeping the vehicle stable and steerable. EBS (electronic braking system) adds faster, brake-by-wire control with a pneumatic backup for redundancy. ALB (automatic load-sensing brake force regulation) adjusts braking effort to the vehicle's load, so an empty and a fully laden truck both brake safely.
- What are the two air lines between tractor and trailer?
- One line supplies air to charge the trailer reservoirs and hold its spring brakes released; the other carries the control signal that applies the trailer service brakes in proportion to driver demand. If the supply line is disconnected or loses pressure, the trailer spring brakes apply automatically. A separate electrical connector carries ABS or EBS signalling.
- Where does the air dryer sit in the diagram?
- Between the compressor and the reservoirs. Placing it there means only dried air reaches the protection valve, the service valves and the brake chambers. If the dryer is bypassed or its cartridge is saturated, moisture passes downstream and causes valve corrosion in service and line icing in winter.
- Why is a relay valve used instead of running long lines to each chamber?
- Air is compressible, so a long line between the foot brake valve and a distant chamber would delay application. A relay valve is mounted close to the axle and uses the driver's signal only as a command, admitting reservoir air locally. This shortens both application and release time, which matters most on the rear axle and on trailers.
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