Pneumatic System

Air Brake System —
Operation, Diagnostics & Repair

Pneumatic circuits, braking logic, and failure analysis. All values in metric/SI standard.

System Overview Principle of Operation Brake Chambers Air Circuit Diagnostics Failure Modes Repair Guidelines Safety Notes

The air brake system uses compressed air to actuate service brakes and spring-applied parking brakes. It is the standard braking system for trucks and buses due to its reliability, fail-safe characteristics, and ability to operate trailer brakes.

Compressed Air Generation

The engine-driven compressor generates approximately 8–12 bar system pressure. The air dryer removes moisture to prevent freezing and corrosion in the pneumatic circuit.

Dual-Circuit Safety

Primary circuit

Supplies rear axle brakes

Secondary circuit

Supplies front axle brakes

Failure of one circuit must not cause total brake loss. This is a fundamental safety requirement.

Braking Sequence

1.Driver presses brake pedal

2.Foot brake valve meters control pressure proportional to pedal force

3.Control pressure reaches relay valve

4.Relay valve rapidly delivers reservoir air to brake chambers

5.Brake chamber converts air pressure into pushrod force (F = P × A)

6.Pushrod actuates slack adjuster / S-cam or disc actuator

7.Brakes apply — vehicle decelerates

System Pressure Build-Up

Start engine and monitor time to reach approximately 10 bar on the system gauge.

Build-up within manufacturer specification (typically 3–5 minutes)

Slow build-up = Compressor issue, air dryer restriction, or system leak

Leak Test

Engine OFF. Monitor system pressure drop over time with brakes released and then applied.

Acceptable leak rate: below 0.5 bar/min

Higher = System leak — isolate and locate with soap solution or ultrasonic detector

Brake Application Test

Apply service brake fully and observe pressure drop in reservoirs.

Moderate, consistent pressure drop

Large drop = Insufficient reservoir volume, leak, or restriction in circuit

Relay Valve Function

Check delay between pedal input and brake response at each wheel.

Near-instantaneous response at all wheels

Delay = Faulty relay valve, restriction, contamination, or frozen line

Brake Chamber Stroke

Measure pushrod travel at each brake chamber during full brake application.

Stroke within OEM specification

Excessive stroke = Worn brakes, slack adjuster issue, or incorrect adjustment

Excessive stroke can lead to brake fade and potential brake failure.

Spring Brake Test

Apply and release parking brake. Verify proper spring brake engagement and release.

Parking brake holds vehicle securely on grade

Release is complete — no residual drag

Never attempt to disassemble a spring brake chamber without first caging the spring.

Air Brake System —Operation, Diagnostics & Repair

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