How to Diagnose Air Loss in Truck & Trailer Pneumatic Systems

A complete step-by-step guide for diagnosing air loss in truck and semi-trailer pneumatic systems — covering air tanks, valves, hoses, brake chambers, governor tests and leak-down procedures.

📅 Published on 2025-12-17 | ✍️ Semi Trailer News Technical Desk

Image: Technician performing air leak inspection on tractor + trailer pneumatic system

🔍 What Causes Air Loss in Pneumatic Systems?

Air loss in heavy trucks and semi trailers originates from leaks in hoses, brake chambers, valves, air tanks, fittings, quick releases and suspension bellows. A slow leak may take 15–30 minutes to drop from 10 bar to 7 bar, while a severe leak may empty the system in minutes.

• Damaged nylon hoses
• Loose OR damaged push-in fittings
• Air tank drain valve leaks
• Worn or cracked air bellows
• Brake chamber diaphragm rupture
• Faulty relay valve / protection valve
• Defective air dryer purge valve
• Compressor governor out of calibration

🧪 Step 1 — Perform a Leak-Down Test

Standard method used by fleets and DOT/EC inspectors:

1. Charge system to 10–12 bar.
2. Switch engine OFF.
3. Apply full service brake for 1 minute.
4. Record pressure drop.

Acceptable Leak Limits:

• Tractor only: ≤ 0.35 bar per minute
• Tractor + Trailer: ≤ 0.50 bar per minute

If leak exceeds limits → proceed to isolation tests.

🔧 Step 2 — Isolate Tractor vs Trailer

Use the following procedure:

1. Disconnect red supply gladhand
2. Install a gladhand blank tester or ball valve
3. Pressurize tractor → observe drop
4. Reconnect and repeat for blue service line

This identifies whether the leak is on the tractor side or trailer side.

🛠 Step 3 — Check Common Leak Points

• Push-in fittings: Spray soapy water and look for bubbles
• Brake chambers: Listen for hissing around diaphragm
• Relay valves: Check exhaust ports
• Air dryer: Inspect purge valve for constant leaking
• Suspension: Look for cracked air bellows
• Air tanks: Test drain valves and rusted areas

🧮 Quick Pressure Drop Calculator

⚠️ Step 4 — Signs of Dangerous Air Loss

• Air dryer purges repeatedly every 10–20 seconds
• Brake warning light activates during idle
• Compressor runs continuously
• Trailer park brake releases slowly
• Suspension struggles to maintain ride height

🔩 Step 5 — Fix the Root Cause

Typical repair actions:

• Replace damaged air hoses
• Renew push-in fittings (6mm / 8mm / 12mm)
• Replace leaking relay valve
• Install new brake chamber diaphragm
• Rebuild air dryer purge valve
• Adjust or replace faulty compressor governor

🏁 Conclusion

Diagnosing air loss requires a structured procedure: leak-down test → isolation → targeted inspection. Correct troubleshooting prevents roadside breakdowns, brake failure risks, and excessive compressor wear.

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