Air Brake System Diagram: How Every Component Connects
Air Brake Systems

Air Brake System Diagram: How Every Component Connects

Vaden Team
Vaden Team

Temmuz 10, 2026

An air brake system diagram makes it easy to see how compressed air travels from the compressor all the way to the wheels of a truck, bus or trailer. Below is a simplified schematic of a heavy-vehicle air brake system, followed by a plain-English explanation of every component and how they connect.

Truck Air Brake System — How the Components Connect Air supply (charging) Brake application (service) ABS / electronic Air Compressor Governor Air Dryer Air Reservoirswet · primary · secondary compressed air supply to brake valves Foot BrakeValve Relay / QR Valve Brake Chamber Slack Adjusterforce multiplier S-Cam &Brake Drum ABS Modulator Wheel Speed Sensor Simplified schematic — VADEN ORIGINAL. Trailer circuit adds a tractor protection valve, gladhands and a trailer control valve.

The air brake system, component by component

The system has two circuits: an air supply (charging) circuit shown in blue, and a brake application (service) circuit shown in red.

Air supply circuit

  • Air compressor — engine-driven pump that builds the compressed air the whole system runs on.
  • Governor — controls the compressor's cut-in and cut-out pressure so the tanks stay in the correct range.
  • Air dryer — removes moisture and oil so valves do not corrode, stick or freeze. The unloader / pressure regulator works alongside it.
  • Air reservoirs — the wet, primary and secondary tanks store energy as compressed air and keep the two service circuits independent for safety.

Brake application circuit

  • Foot brake valve — the driver's demand valve; it meters air to the service line in proportion to pedal effort.
  • Relay valve and quick release valve — speed up application and release at the rear axles by venting or feeding air locally instead of all the way back to the pedal.
  • Brake chamber — converts air pressure into a mechanical push. Service chambers apply the brakes; spring (park) chambers hold the vehicle and apply automatically if pressure is lost.
  • Slack adjuster — a lever that multiplies the chamber force and keeps the lining-to-drum clearance correct.
  • S-cam & brake drum — the slack adjuster rotates the S-cam, which forces the shoes against the drum (disc air brakes use a caliper instead).

Load and levelling control

A load sensing valve (ALB) adjusts braking force to the vehicle's load, while a levelling valve keeps air-suspension ride height correct. Many trucks also add an exhaust brake to slow the vehicle without using the service brakes.

Where ABS fits into the air brake diagram

Anti-lock braking sits on top of the service circuit. A wheel speed sensor at each wheel feeds the ECU; if a wheel is about to lock, the ABS modulator valve rapidly releases and re-applies air to that chamber to keep the wheel rolling and the vehicle steerable.

Trailer connection

On a tractor-trailer, air crosses to the trailer through a tractor protection valve and gladhand couplings, feeding the trailer's own relay valve, reservoirs and brake chambers — a mirror of the diagram above.

Frequently Asked Questions

What are the main components of an air brake system?

The core components are the air compressor, governor, air dryer, air reservoirs, foot brake valve, relay and quick release valves, brake chambers, slack adjusters and the S-cam or disc brake at each wheel, plus the ABS modulator and wheel speed sensors.

How does air flow through an air brake system?

The compressor builds pressure, the air dryer cleans it, and the reservoirs store it. When the driver presses the foot valve, air is metered to the relay valves and brake chambers, which push the slack adjusters and S-cams to apply the brakes.

What is the difference between the service and parking (spring) brakes?

The service brake applies air pressure to stop the vehicle. The parking or spring brake uses a powerful spring that is held off by air; if air pressure is lost, the spring applies automatically, making the system fail-safe.

Where does ABS fit into the air brake diagram?

ABS sits on the service circuit. Wheel speed sensors detect impending lock-up and the ABS modulator valve rapidly modulates air to each brake chamber, keeping the wheels rolling under hard braking.

What is the difference between drum and disc air brakes?

Drum air brakes use an S-cam to press shoes against a rotating drum; disc air brakes use an air-actuated caliper to clamp a rotor. Both are fed by the same air supply circuit shown in the diagram.

air brake system diagram air brake components abs truck brakes
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